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2015-11-03atomic: remove all traces of READ_ONCE_CTRL() and atomic*_read_ctrl()Linus Torvalds
This seems to be a mis-reading of how alpha memory ordering works, and is not backed up by the alpha architecture manual. The helper functions don't do anything special on any other architectures, and the arguments that support them being safe on other architectures also argue that they are safe on alpha. Basically, the "control dependency" is between a previous read and a subsequent write that is dependent on the value read. Even if the subsequent write is actually done speculatively, there is no way that such a speculative write could be made visible to other cpu's until it has been committed, which requires validating the speculation. Note that most weakely ordered architectures (very much including alpha) do not guarantee any ordering relationship between two loads that depend on each other on a control dependency: read A if (val == 1) read B because the conditional may be predicted, and the "read B" may be speculatively moved up to before reading the value A. So we require the user to insert a smp_rmb() between the two accesses to be correct: read A; if (A == 1) smp_rmb() read B Alpha is further special in that it can break that ordering even if the *address* of B depends on the read of A, because the cacheline that is read later may be stale unless you have a memory barrier in between the pointer read and the read of the value behind a pointer: read ptr read offset(ptr) whereas all other weakly ordered architectures guarantee that the data dependency (as opposed to just a control dependency) will order the two accesses. As a result, alpha needs a "smp_read_barrier_depends()" in between those two reads for them to be ordered. The coontrol dependency that "READ_ONCE_CTRL()" and "atomic_read_ctrl()" had was a control dependency to a subsequent *write*, however, and nobody can finalize such a subsequent write without having actually done the read. And were you to write such a value to a "stale" cacheline (the way the unordered reads came to be), that would seem to lose the write entirely. So the things that make alpha able to re-order reads even more aggressively than other weak architectures do not seem to be relevant for a subsequent write. Alpha memory ordering may be strange, but there's no real indication that it is *that* strange. Also, the alpha architecture reference manual very explicitly talks about the definition of "Dependence Constraints" in section 5.6.1.7, where a preceding read dominates a subsequent write. Such a dependence constraint admittedly does not impose a BEFORE (alpha architecture term for globally visible ordering), but it does guarantee that there can be no "causal loop". I don't see how you could avoid such a loop if another cpu could see the stored value and then impact the value of the first read. Put another way: the read and the write could not be seen as being out of order wrt other cpus. So I do not see how these "x_ctrl()" functions can currently be necessary. I may have to eat my words at some point, but in the absense of clear proof that alpha actually needs this, or indeed even an explanation of how alpha could _possibly_ need it, I do not believe these functions are called for. And if it turns out that alpha really _does_ need a barrier for this case, that barrier still should not be "smp_read_barrier_depends()". We'd have to make up some new speciality barrier just for alpha, along with the documentation for why it really is necessary. Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Paul E McKenney <paulmck@us.ibm.com> Cc: Dmitry Vyukov <dvyukov@google.com> Cc: Will Deacon <will.deacon@arm.com> Cc: Ingo Molnar <mingo@kernel.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2015-11-03Merge branch 'locking-core-for-linus' of ↵Linus Torvalds
git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip Pull locking changes from Ingo Molnar: "The main changes in this cycle were: - More gradual enhancements to atomic ops: new atomic*_read_ctrl() ops, synchronize atomic_{read,set}() ordering requirements between architectures, add atomic_long_t bitops. (Peter Zijlstra) - Add _{relaxed|acquire|release}() variants for inc/dec atomics and use them in various locking primitives: mutex, rtmutex, mcs, rwsem. This enables weakly ordered architectures (such as arm64) to make use of more locking related optimizations. (Davidlohr Bueso) - Implement atomic[64]_{inc,dec}_relaxed() on ARM. (Will Deacon) - Futex kernel data cache footprint micro-optimization. (Rasmus Villemoes) - pvqspinlock runtime overhead micro-optimization. (Waiman Long) - misc smaller fixlets" * 'locking-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: ARM, locking/atomics: Implement _relaxed variants of atomic[64]_{inc,dec} locking/rwsem: Use acquire/release semantics locking/mcs: Use acquire/release semantics locking/rtmutex: Use acquire/release semantics locking/mutex: Use acquire/release semantics locking/asm-generic: Add _{relaxed|acquire|release}() variants for inc/dec atomics atomic: Implement atomic_read_ctrl() atomic, arch: Audit atomic_{read,set}() atomic: Add atomic_long_t bitops futex: Force hot variables into a single cache line locking/pvqspinlock: Kick the PV CPU unconditionally when _Q_SLOW_VAL locking/osq: Relax atomic semantics locking/qrwlock: Rename ->lock to ->wait_lock locking/Documentation/lockstat: Fix typo - lokcing -> locking locking/atomics, cmpxchg: Privatize the inclusion of asm/cmpxchg.h
2015-10-06documentation: Add lockless_dereference()Paul E. McKenney
The recently added lockless_dereference() macro is not present in the Documentation/ directory, so this commit fixes that. Reported-by: Dmitry Vyukov <dvyukov@google.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2015-10-06documentation: No acquire/release for RCU readersPaul E. McKenney
Documentation/memory-barriers.txt calls out RCU as one of the sets of primitives associated with ACQUIRE and RELEASE. There really is an association in that rcu_assign_pointer() includes a RELEASE operation, but a quick read can convince people that rcu_read_lock() and rcu_read_unlock() have ACQUIRE and RELEASE semantics, which they do not. This commit therefore removes RCU from this list in order to avoid this confusion. Reported-by: Boqun Feng <boqun.feng@gmail.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2015-09-23atomic: Implement atomic_read_ctrl()Peter Zijlstra
Provide atomic_read_ctrl() to mirror READ_ONCE_CTRL(), such that we can more conveniently use atomics in control dependencies. Since we can assume atomic_read() implies a READ_ONCE(), we must only emit an extra smp_read_barrier_depends() in order to upgrade to READ_ONCE_CTRL() semantics. Requested-by: Dmitry Vyukov <dvyukov@google.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Will Deacon <will.deacon@arm.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: linux-kernel@vger.kernel.org Cc: oleg@redhat.com Link: http://lkml.kernel.org/r/20150918115637.GM3604@twins.programming.kicks-ass.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2015-09-03Merge branch 'locking-core-for-linus' of ↵Linus Torvalds
git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip Pull locking and atomic updates from Ingo Molnar: "Main changes in this cycle are: - Extend atomic primitives with coherent logic op primitives (atomic_{or,and,xor}()) and deprecate the old partial APIs (atomic_{set,clear}_mask()) The old ops were incoherent with incompatible signatures across architectures and with incomplete support. Now every architecture supports the primitives consistently (by Peter Zijlstra) - Generic support for 'relaxed atomics': - _acquire/release/relaxed() flavours of xchg(), cmpxchg() and {add,sub}_return() - atomic_read_acquire() - atomic_set_release() This came out of porting qwrlock code to arm64 (by Will Deacon) - Clean up the fragile static_key APIs that were causing repeat bugs, by introducing a new one: DEFINE_STATIC_KEY_TRUE(name); DEFINE_STATIC_KEY_FALSE(name); which define a key of different types with an initial true/false value. Then allow: static_branch_likely() static_branch_unlikely() to take a key of either type and emit the right instruction for the case. To be able to know the 'type' of the static key we encode it in the jump entry (by Peter Zijlstra) - Static key self-tests (by Jason Baron) - qrwlock optimizations (by Waiman Long) - small futex enhancements (by Davidlohr Bueso) - ... and misc other changes" * 'locking-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (63 commits) jump_label/x86: Work around asm build bug on older/backported GCCs locking, ARM, atomics: Define our SMP atomics in terms of _relaxed() operations locking, include/llist: Use linux/atomic.h instead of asm/cmpxchg.h locking/qrwlock: Make use of _{acquire|release|relaxed}() atomics locking/qrwlock: Implement queue_write_unlock() using smp_store_release() locking/lockref: Remove homebrew cmpxchg64_relaxed() macro definition locking, asm-generic: Add _{relaxed|acquire|release}() variants for 'atomic_long_t' locking, asm-generic: Rework atomic-long.h to avoid bulk code duplication locking/atomics: Add _{acquire|release|relaxed}() variants of some atomic operations locking, compiler.h: Cast away attributes in the WRITE_ONCE() magic locking/static_keys: Make verify_keys() static jump label, locking/static_keys: Update docs locking/static_keys: Provide a selftest jump_label: Provide a self-test s390/uaccess, locking/static_keys: employ static_branch_likely() x86, tsc, locking/static_keys: Employ static_branch_likely() locking/static_keys: Add selftest locking/static_keys: Add a new static_key interface locking/static_keys: Rework update logic locking/static_keys: Add static_key_{en,dis}able() helpers ...
2015-08-04rcu,locking: Privatize smp_mb__after_unlock_lock()Paul E. McKenney
RCU is the only thing that uses smp_mb__after_unlock_lock(), and is likely the only thing that ever will use it, so this commit makes this macro private to RCU. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Will Deacon <will.deacon@arm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>
2015-08-03locking/Documentation: Clarify failed cmpxchg() memory ordering semanticsWill Deacon
A failed cmpxchg does not provide any memory ordering guarantees, a property that is used to optimise the cmpxchg implementations on Alpha, PowerPC and arm64. This patch updates atomic_ops.txt and memory-barriers.txt to reflect this. Signed-off-by: Will Deacon <will.deacon@arm.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Davidlohr Bueso <dave@stgolabs.net> Cc: Douglas Hatch <doug.hatch@hp.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Scott J Norton <scott.norton@hp.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Waiman Long <waiman.long@hp.com> Link: http://lkml.kernel.org/r/20150716151006.GH26390@arm.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
2015-07-15doc: Call out smp_mb__after_unlock_lock() transitivityPaul E. McKenney
Although "full barrier" should be interpreted as providing transitivity, it is worth eliminating any possible confusion. This commit therefore adds "(including transitivity)" to eliminate any possible confusion. Reported-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-15documentation: Replace ACCESS_ONCE() by READ_ONCE() and WRITE_ONCE()Paul E. McKenney
Reported-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-15documentation: Fix variable-name typo in memory-barriers.txtPaul E. McKenney
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-06-22Merge branch 'locking-core-for-linus' of ↵Linus Torvalds
git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip Pull locking updates from Ingo Molnar: "The main changes are: - 'qspinlock' support, enabled on x86: queued spinlocks - these are now the spinlock variant used by x86 as they outperform ticket spinlocks in every category. (Waiman Long) - 'pvqspinlock' support on x86: paravirtualized variant of queued spinlocks. (Waiman Long, Peter Zijlstra) - 'qrwlock' support, enabled on x86: queued rwlocks. Similar to queued spinlocks, they are now the variant used by x86: CONFIG_ARCH_USE_QUEUED_SPINLOCKS=y CONFIG_QUEUED_SPINLOCKS=y CONFIG_ARCH_USE_QUEUED_RWLOCKS=y CONFIG_QUEUED_RWLOCKS=y - various lockdep fixlets - various locking primitives cleanups, further WRITE_ONCE() propagation" * 'locking-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (24 commits) locking/lockdep: Remove hard coded array size dependency locking/qrwlock: Don't contend with readers when setting _QW_WAITING lockdep: Do not break user-visible string locking/arch: Rename set_mb() to smp_store_mb() locking/arch: Add WRITE_ONCE() to set_mb() rtmutex: Warn if trylock is called from hard/softirq context arch: Remove __ARCH_HAVE_CMPXCHG locking/rtmutex: Drop usage of __HAVE_ARCH_CMPXCHG locking/qrwlock: Rename QUEUE_RWLOCK to QUEUED_RWLOCKS locking/pvqspinlock: Rename QUEUED_SPINLOCK to QUEUED_SPINLOCKS locking/pvqspinlock: Replace xchg() by the more descriptive set_mb() locking/pvqspinlock, x86: Enable PV qspinlock for Xen locking/pvqspinlock, x86: Enable PV qspinlock for KVM locking/pvqspinlock, x86: Implement the paravirt qspinlock call patching locking/pvqspinlock: Implement simple paravirt support for the qspinlock locking/qspinlock: Revert to test-and-set on hypervisors locking/qspinlock: Use a simple write to grab the lock locking/qspinlock: Optimize for smaller NR_CPUS locking/qspinlock: Extract out code snippets for the next patch locking/qspinlock: Add pending bit ...
2015-05-27Merge branches 'array.2015.05.27a', 'doc.2015.05.27a', 'fixes.2015.05.27a', ↵Paul E. McKenney
'hotplug.2015.05.27a', 'init.2015.05.27a', 'tiny.2015.05.27a' and 'torture.2015.05.27a' into HEAD array.2015.05.27a: Remove all uses of RCU-protected array indexes. doc.2015.05.27a: Docuemntation updates. fixes.2015.05.27a: Miscellaneous fixes. hotplug.2015.05.27a: CPU-hotplug updates. init.2015.05.27a: Initialization/Kconfig updates. tiny.2015.05.27a: Updates to Tiny RCU. torture.2015.05.27a: Torture-testing updates.
2015-05-27smp: Make control dependencies work on Alpha, improve documentationPaul E. McKenney
The current formulation of control dependencies fails on DEC Alpha, which does not respect dependencies of any kind unless an explicit memory barrier is provided. This means that the current fomulation of control dependencies fails on Alpha. This commit therefore creates a READ_ONCE_CTRL() that has the same overhead on non-Alpha systems, but causes Alpha to produce the needed ordering. This commit also applies READ_ONCE_CTRL() to the one known use of control dependencies. Use of READ_ONCE_CTRL() also has the beneficial effect of adding a bit of self-documentation to control dependencies. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
2015-05-27documentation: memory-barriers: Fix smp_mb__before_spinlock() semanticsWill Deacon
Our current documentation claims that, when followed by an ACQUIRE, smp_mb__before_spinlock() orders prior loads against subsequent loads and stores, which isn't the intent. This commit therefore fixes the documentation to state that this sequence orders only prior stores against subsequent loads and stores. In addition, the original intent of smp_mb__before_spinlock() was to only order prior loads against subsequent stores, however, people have started using it as if it ordered prior loads against subsequent loads and stores. This commit therefore also updates smp_mb__before_spinlock()'s header comment to reflect this new reality. Cc: Oleg Nesterov <oleg@redhat.com> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Will Deacon <will.deacon@arm.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-05-19locking/arch: Rename set_mb() to smp_store_mb()Peter Zijlstra
Since set_mb() is really about an smp_mb() -- not a IO/DMA barrier like mb() rename it to match the recent smp_load_acquire() and smp_store_release(). Suggested-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: linux-kernel@vger.kernel.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
2015-04-18Merge tag 'docs-for-linus' of git://git.lwn.net/linux-2.6Linus Torvalds
Pull documentation updates from Jonathan Corbet: "Numerous fixes, the overdue removal of the i2o docs, some new Chinese translations, and, hopefully, the README fix that will end the flow of identical patches to that file" * tag 'docs-for-linus' of git://git.lwn.net/linux-2.6: (34 commits) Documentation/memcg: update memcg/kmem status Documentation: blackfin: Makefile: Typo building issue Documentation/vm/pagemap.txt: correct location of page-types tool Documentation/memory-barriers.txt: typo fix doc: Add guest_nice column to example output of `cat /proc/stat' Documentation/kernel-parameters: Move "eagerfpu" to its right place Documentation: gpio: Update ACPI part of the document to mention _DSD docs/completion.txt: Various tweaks and corrections doc: completion: context, scope and language fixes Documentation:Update Documentation/zh_CN/arm64/memory.txt Documentation:Update Documentation/zh_CN/arm64/booting.txt Documentation: Chinese translation of arm64/legacy_instructions.txt DocBook media: fix broken EIA hyperlink Documentation: tweak the maintainers entry README: Change gzip/bzip2 to xz compression format README: Update version number reference doc:pci: Fix typo in Documentation/PCI Documentation: drm: Use '->' when describing access through pointers. Documentation: Remove mentioning of block barriers Documentation/email-clients.txt: Fix one grammar mistake, add extra info about TB ...
2015-04-08Documentation/memory-barriers.txt: typo fixSylvain Trias
Fix an obvious typo in the documentation. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
2015-02-26documentation: Clarify control-dependency pairingPaul E. McKenney
This commit explicitly states that control dependencies pair normally with other barriers, and gives an example of such pairing. Reported-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
2015-01-07documentation: Fix smp typo in memory-barriers.txtDavidlohr Bueso
Signed-off-by: Davidlohr Bueso <dbueso@suse.de> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-01-07documentation: Record limitations of bitfields and small variablesPaul E. McKenney
This commit documents the fact that it is not safe to use bitfields as shared variables in synchronization algorithms. It also documents that CPUs must be able to concurrently load from and store to adjacent one-byte and two-byte variables, which is in fact required by the C11 standard (Section 3.14). Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-12-11arch: Add lightweight memory barriers dma_rmb() and dma_wmb()Alexander Duyck
There are a number of situations where the mandatory barriers rmb() and wmb() are used to order memory/memory operations in the device drivers and those barriers are much heavier than they actually need to be. For example in the case of PowerPC wmb() calls the heavy-weight sync instruction when for coherent memory operations all that is really needed is an lsync or eieio instruction. This commit adds a coherent only version of the mandatory memory barriers rmb() and wmb(). In most cases this should result in the barrier being the same as the SMP barriers for the SMP case, however in some cases we use a barrier that is somewhere in between rmb() and smp_rmb(). For example on ARM the rmb barriers break down as follows: Barrier Call Explanation --------- -------- ---------------------------------- rmb() dsb() Data synchronization barrier - system dma_rmb() dmb(osh) data memory barrier - outer sharable smp_rmb() dmb(ish) data memory barrier - inner sharable These new barriers are not as safe as the standard rmb() and wmb(). Specifically they do not guarantee ordering between coherent and incoherent memories. The primary use case for these would be to enforce ordering of reads and writes when accessing coherent memory that is shared between the CPU and a device. It may also be noted that there is no dma_mb(). Most architectures don't provide a good mechanism for performing a coherent only full barrier without resorting to the same mechanism used in mb(). As such there isn't much to be gained in trying to define such a function. Cc: Frederic Weisbecker <fweisbec@gmail.com> Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> Cc: Michael Ellerman <michael@ellerman.id.au> Cc: Michael Neuling <mikey@neuling.org> Cc: Russell King <linux@arm.linux.org.uk> Cc: Geert Uytterhoeven <geert@linux-m68k.org> Cc: Heiko Carstens <heiko.carstens@de.ibm.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> Cc: Tony Luck <tony.luck@intel.com> Cc: Oleg Nesterov <oleg@redhat.com> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Ingo Molnar <mingo@kernel.org> Cc: David Miller <davem@davemloft.net> Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org> Acked-by: Will Deacon <will.deacon@arm.com> Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
2014-12-09Merge branch 'core-rcu-for-linus' of ↵Linus Torvalds
git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip Pull RCU updates from Ingo Molnar: "These are the main changes in this cycle: - Streamline RCU's use of per-CPU variables, shifting from "cpu" arguments to functions to "this_"-style per-CPU variable accessors. - signal-handling RCU updates. - real-time updates. - torture-test updates. - miscellaneous fixes. - documentation updates" * 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (34 commits) rcu: Fix FIXME in rcu_tasks_kthread() rcu: More info about potential deadlocks with rcu_read_unlock() rcu: Optimize cond_resched_rcu_qs() rcu: Add sparse check for RCU_INIT_POINTER() documentation: memory-barriers.txt: Correct example for reorderings documentation: Add atomic_long_t to atomic_ops.txt documentation: Additional restriction for control dependencies documentation: Document RCU self test boot params rcutorture: Fix rcu_torture_cbflood() memory leak rcutorture: Remove obsolete kversion param in kvm.sh rcutorture: Remove stale test configurations rcutorture: Enable RCU self test in configs rcutorture: Add early boot self tests torture: Run Linux-kernel binary out of results directory cpu: Avoid puts_pending overflow rcu: Remove "cpu" argument to rcu_cleanup_after_idle() rcu: Remove "cpu" argument to rcu_prepare_for_idle() rcu: Remove "cpu" argument to rcu_needs_cpu() rcu: Remove "cpu" argument to rcu_note_context_switch() rcu: Remove "cpu" argument to rcu_preempt_check_callbacks() ...
2014-11-13documentation: memory-barriers.txt: Correct example for reorderingsPranith Kumar
Correct the example of memory orderings in memory-barriers.txt Commit 615cc2c9cf95 "Documentation/memory-barriers.txt: fix important typo re memory barriers" changed the assignment to x and y. Change the rest of the example to match this change. Reported-by: Ganesh Rapolu <ganesh.rapolu@hotmail.com> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-11-13documentation: Additional restriction for control dependenciesPaul E. McKenney
Short-circuit booleans are not defences against compilers breaking your intended control dependencies. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
2014-10-20documentation: memory-barriers: clarify relaxed io accessor semanticsWill Deacon
This patch extends the paragraph describing the relaxed read io accessors so that the relaxed accessors are defined to be: - Ordered with respect to each other if accessing the same peripheral - Unordered with respect to normal memory accesses - Unordered with respect to LOCK/UNLOCK operations Whilst many architectures will provide stricter semantics, ARM, Alpha and PPC can achieve significant performance gains by taking advantage of some or all of the above relaxations. Cc: Randy Dunlap <rdunlap@infradead.org> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: David Howells <dhowells@redhat.com> Signed-off-by: Will Deacon <will.deacon@arm.com>
2014-09-07memory-barriers: Fix description of 2-legged-if-based control dependenciesPaul E. McKenney
Sad to say, current compilers really will hoist identical stores from both branches of an "if" statement to precede the conditional. This commit therefore updates the description of control dependencies to reflect this ugly reality. Reported-by: Pranith Kumar <bobby.prani@gmail.com> Reported-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-09-07memory-barriers: Retain barrier() in fold-to-zero examplePaul E. McKenney
The transformation in the fold-to-zero example incorrectly omits the barrier() directive. This commit therefore adds it back in. Reported-by: Pranith Kumar <pranith@gatech.edu> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-09-07memory-barriers: Fix control-ordering no-transitivity examplePaul E. McKenney
The control-ordering example demonstrating lack of transitivity had multiple problems. This commit fixes them. Reported-by: Nikolay Samofatov <nikolay.samofatov@gmail.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
2014-07-08documentation: Add acquire/release barriers to pairing rulesPaul E. McKenney
It is possible to pair acquire and release barriers with other barriers, so this commit adds them to the list in the SMP barrier pairing section. Reported-by: Lai Jiangshan <laijs@cn.fujitsu.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Tejun Heo <tj@kernel.org> Reviewed-by: Josh Triplett <josh@joshtriplett.org> [ paulmck: Updated pairing discussion as suggested by Peter Zijlstra. ]
2014-07-08documentation: Clarify wake-up/memory-barrier relationshipPaul E. McKenney
This commit adds an example demonstrating that if a wake_up() doesn't actually wake something up, no memory ordering is provided. Reported-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org> Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com> Acked-by: Peter Zijlstra <peterz@infradead.org>
2014-06-06Documentation/memory-barriers.txt: fix important typo re memory barriersAlexey Dobriyan
Examples introducing neccesity of RMB+WMP pair reads as A=3 READ B www rrrrrr B=4 READ A Note the opposite order of reads vs writes. But the first example without barriers reads as A=3 READ A B=4 READ B There are 4 outcomes in the first example. But if someone new to the concept tries to insert barriers like this: A=3 READ A www rrrrrr B=4 READ B he will still get all 4 possible outcomes, because "READ A" is first. All this can be utterly confusing because barrier pair seems to be superfluous. In short, fixup first example to match latter examples with barriers. Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com> Cc: David Howells <dhowells@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2014-04-18arch,doc: Convert smp_mb__*()Peter Zijlstra
Update the documentation to reflect the change of barrier primitives. Signed-off-by: Peter Zijlstra <peterz@infradead.org> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Acked-by: David Howells <dhowells@redhat.com> Link: http://lkml.kernel.org/n/tip-xslfehiga1twbk5uk94rij1e@git.kernel.org Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Randy Dunlap <rdunlap@infradead.org> Cc: linux-doc@vger.kernel.org Cc: linux-kernel@vger.kernel.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-04-02Merge branch 'for-linus' of ↵Linus Torvalds
git://git.kernel.org/pub/scm/linux/kernel/git/jikos/trivial Pull trivial tree updates from Jiri Kosina: "Usual rocket science -- mostly documentation and comment updates" * 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/jikos/trivial: sparse: fix comment doc: fix double words isdn: capi: fix "CAPI_VERSION" comment doc: DocBook: Fix typos in xml and template file Bluetooth: add module name for btwilink driver core: unexport static function create_syslog_header mmc: core: typo fix in printk specifier ARM: spear: clean up editing mistake net-sysfs: fix comment typo 'CONFIG_SYFS' doc: Insert MODULE_ in module-signing macros Documentation: update URL to hfsplus Technote 1150 gpio: update path to documentation ixgbe: Fix format string in ixgbe_fcoe. Kconfig: Remove useless "default N" lines user_namespace.c: Remove duplicated word in comment CREDITS: fix formatting treewide: Fix typo in Documentation/DocBook mm: Fix warning on make htmldocs caused by slab.c ata: ata-samsung_cf: cleanup in header file idr: remove unused prototype of idr_free()
2014-03-21doc: fix double wordsMasanari Iida
Fix double words "the the" in various files within Documentations. Signed-off-by: Masanari Iida <standby24x7@gmail.com> Signed-off-by: Jiri Kosina <jkosina@suse.cz>
2014-02-24Documentation/memory-barriers.txt: Clarify release/acquire orderingPaul E. McKenney
This commit fixes a couple of typos and clarifies what happens when the CPU chooses to execute a later lock acquisition before a prior lock release, in particular, why deadlock is avoided. Reported-by: Peter Hurley <peter@hurleysoftware.com> Reported-by: James Bottomley <James.Bottomley@HansenPartnership.com> Reported-by: Stefan Richter <stefanr@s5r6.in-berlin.de> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-02-17Documentation/memory-barriers.txt: Need barriers() for some control dependenciesPaul E. McKenney
Current compilers can "speculate" stores in the case where both legs of the "if" statement start with identical stores. Because the stores are identical, the compiler knows that the store will unconditionally execute regardless of the "if" condition, and so the compiler is within its rights to hoist the store to precede the condition. Such hoisting destroys the control-dependency ordering. This ordering can be restored by placing a barrier() at the beginning of each leg of the "if" statement. This commit adds this requirement to the control-dependencies section. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-02-17Documentation/memory-barriers.txt: Conditional must use prior loadPaul E. McKenney
A control dependency consists of a load, a conditional that depends on that load, and a store. This commit emphasizes this point in the summary. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2014-02-17Documentation/memory-barriers.txt: ACCESS_ONCE() provides cache coherencePaul E. McKenney
The ACCESS_ONCE() primitive provides cache coherence, but the documentation does not clearly state this. This commit therefore upgrades the documentation. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2014-01-12locking/doc: Rename LOCK/UNLOCK to ACQUIRE/RELEASEPeter Zijlstra
The LOCK and UNLOCK barriers as described in our barrier document are generally known as ACQUIRE and RELEASE barriers in other literature. Since we plan to introduce the acquire and release nomenclature in generic kernel primitives we should amend the document to avoid confusion as to what an acquire/release means. Reviewed-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> Signed-off-by: Peter Zijlstra <peterz@infradead.org> Acked-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: Frederic Weisbecker <fweisbec@gmail.com> Cc: Michael Ellerman <michael@ellerman.id.au> Cc: Michael Neuling <mikey@neuling.org> Cc: Russell King <linux@arm.linux.org.uk> Cc: Geert Uytterhoeven <geert@linux-m68k.org> Cc: Heiko Carstens <heiko.carstens@de.ibm.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> Cc: Victor Kaplansky <VICTORK@il.ibm.com> Cc: Tony Luck <tony.luck@intel.com> Cc: Oleg Nesterov <oleg@redhat.com> Link: http://lkml.kernel.org/r/20131217092435.GC21999@twins.programming.kicks-ass.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2013-12-16Documentation/memory-barriers.txt: Downgrade UNLOCK+BLOCKPaul E. McKenney
Historically, an UNLOCK+LOCK pair executed by one CPU, by one task, or on a given lock variable has implied a full memory barrier. In a recent LKML thread, the wisdom of this historical approach was called into question: http://www.spinics.net/lists/linux-mm/msg65653.html, in part due to the memory-order complexities of low-handoff-overhead queued locks on x86 systems. This patch therefore removes this guarantee from the documentation, and further documents how to restore it via a new smp_mb__after_unlock_lock() primitive. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: <linux-arch@vger.kernel.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Link: http://lkml.kernel.org/r/1386799151-2219-6-git-send-email-paulmck@linux.vnet.ibm.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
2013-12-16Documentation/memory-barriers.txt: Document ACCESS_ONCE()Paul E. McKenney
The situations in which ACCESS_ONCE() is required are not well documented, so this commit adds some verbiage to memory-barriers.txt. Reported-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org> Reviewed-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: <linux-arch@vger.kernel.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Link: http://lkml.kernel.org/r/1386799151-2219-4-git-send-email-paulmck@linux.vnet.ibm.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
2013-12-16Documentation/memory-barriers.txt: Prohibit speculative writesPeter Zijlstra
No SMP architecture currently supporting Linux allows speculative writes, so this commit updates Documentation/memory-barriers.txt to prohibit them in Linux core code. It also records restrictions on their use. Signed-off-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org> Reviewed-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: <linux-arch@vger.kernel.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Link: http://lkml.kernel.org/r/1386799151-2219-3-git-send-email-paulmck@linux.vnet.ibm.com [ Paul modified the original patch from Peter. ] Signed-off-by: Ingo Molnar <mingo@kernel.org>
2013-12-16Documentation/memory-barriers.txt: Add long atomic examples to ↵Paul E. McKenney
memory-barriers.txt Although the atomic_long_t functions are quite useful, they are a bit obscure. This commit therefore adds the common ones alongside their atomic_t counterparts in Documentation/memory-barriers.txt. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org> Reviewed-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: <linux-arch@vger.kernel.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Link: http://lkml.kernel.org/r/1386799151-2219-2-git-send-email-paulmck@linux.vnet.ibm.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
2013-12-16Documentation/memory-barriers.txt: Add needed ACCESS_ONCE() calls to ↵Paul E. McKenney
memory-barriers.txt The Documentation/memory-barriers.txt file was written before the need for ACCESS_ONCE() was fully appreciated. It therefore contains no ACCESS_ONCE() calls, which can be a problem when people lift examples from it. This commit therefore adds ACCESS_ONCE() calls. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org> Reviewed-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: <linux-arch@vger.kernel.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Link: http://lkml.kernel.org/r/1386799151-2219-1-git-send-email-paulmck@linux.vnet.ibm.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
2013-11-22Documentation/memory-barriers.txt: Fix a typo in the data dependency descriptionIngo Molnar
This typo has been there forever, it is 7.5 years old, looks like this section of our memory ordering documentation is a place where most eyes are glazed over already ;-) [ Also fix some stray spaces and stray tabs while at it, shrinking the file by 49 bytes. Visual output unchanged. ] Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Link: http://lkml.kernel.org/n/tip-gncea9cb8igosblizfqMXrie@git.kernel.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
2013-08-19doc: Fix memory-barrier control-dependency examplePaul E. McKenney
Each control-dependency example needs its barriers between the "if" condition and the body of the "if" because a control dependency is a dependency induced by a branch. This commit makes the needed adjustment. Reported-by: Yongming Shen <symingz@gmail.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2013-01-08Documentation: Memory barrier semantics of atomic_xchg()Richard Braun
Add atomic_xchg() to documentation for atomic operations and memory barriers. Signed-off-by: Richard Braun <rbraun@sceen.net> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2012-10-23Documentation: Fix memory-barriers.txt examplePaul E. McKenney
This commit fixes a broken example of overlapping stores in the Documentation/memory-barriers.txt file. Reported-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2011-09-27doc: fix broken referencesPaul Bolle
There are numerous broken references to Documentation files (in other Documentation files, in comments, etc.). These broken references are caused by typo's in the references, and by renames or removals of the Documentation files. Some broken references are simply odd. Fix these broken references, sometimes by dropping the irrelevant text they were part of. Signed-off-by: Paul Bolle <pebolle@tiscali.nl> Signed-off-by: Jiri Kosina <jkosina@suse.cz>