linux-toradex.git/include/linux/futex.h, branch v2.6.26-rc1

futex: runtime enable pi and robust functionality

2008-02-24T01:12:15+00:00

Not all architectures implement futex_atomic_cmpxchg_inatomic().  The default
implementation returns -ENOSYS, which is currently not handled inside of the
futex guts.

Futex PI calls and robust list exits with a held futex result in an endless
loop in the futex code on architectures which have no support.

Fixing up every place where futex_atomic_cmpxchg_inatomic() is called would
add a fair amount of extra if/else constructs to the already complex code.  It
is also not possible to disable the robust feature before user space tries to
register robust lists.

Compile time disabling is not a good idea either, as there are already
architectures with runtime detection of futex_atomic_cmpxchg_inatomic support.

Detect the functionality at runtime instead by calling
cmpxchg_futex_value_locked() with a NULL pointer from the futex initialization
code.  This is guaranteed to fail, but the call of
futex_atomic_cmpxchg_inatomic() happens with pagefaults disabled.

On architectures, which use the asm-generic implementation or have a runtime
CPU feature detection, a -ENOSYS return value disables the PI/robust features.

On architectures with a working implementation the call returns -EFAULT and
the PI/robust features are enabled.

The relevant syscalls return -ENOSYS and the robust list exit code is blocked,
when the detection fails.

Fixes http://lkml.org/lkml/2008/2/11/149
Originally reported by: Lennart Buytenhek

Signed-off-by: Thomas Gleixner 
Acked-by: Ingo Molnar 
Cc: Lennert Buytenhek 
Cc: Riku Voipio 
Cc: 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

futex: Add bitset conditional wait/wakeup functionality

2008-02-01T16:45:14+00:00

To allow the implementation of optimized rw-locks in user space, glibc
needs a possibility to select waiters for wakeup depending on a bitset
mask.

This requires two new futex OPs: FUTEX_WAIT_BITS and FUTEX_WAKE_BITS
These OPs are basically the same as FUTEX_WAIT and FUTEX_WAKE plus an
additional argument - a bitset. Further the FUTEX_WAIT_BITS OP is
expecting an absolute timeout value instead of the relative one, which
is used for the FUTEX_WAIT OP.

FUTEX_WAIT_BITS calls into the kernel with a bitset. The bitset is
stored in the futex_q structure, which is used to enqueue the waiter
into the hashed futex waitqueue.

FUTEX_WAKE_BITS also calls into the kernel with a bitset. The wakeup
function logically ANDs the bitset with the bitset stored in each
waiters futex_q structure. If the result is zero (i.e. none of the set
bits in the bitsets is matching), then the waiter is not woken up. If
the result is not zero (i.e. one of the set bits in the bitsets is
matching), then the waiter is woken.

The bitset provided by the caller must be non zero. In case the
provided bitset is zero the kernel returns EINVAL.

Internaly the new OPs are only extensions to the existing FUTEX_WAIT
and FUTEX_WAKE functions. The existing OPs hand a bitset with all bits
set into the futex_wait() and futex_wake() functions.

Signed-off-by: Thomas Gleixner 
Signed-off-by: Ingo Molnar

sched, futex: detach sched.h and futex.h

2008-01-25T20:08:34+00:00

Signed-off-by: Alexey Dobriyan 
Signed-off-by: Ingo Molnar

kernel/futex.c: make 3 functions static

2007-11-05T10:53:46+00:00

The following functions can now become static again:
- get_futex_key()
- get_futex_key_refs()
- drop_futex_key_refs()

Signed-off-by: Adrian Bunk 
Signed-off-by: Rusty Russell

Revert "futex_requeue_pi optimization"

2007-06-18T16:48:41+00:00

This reverts commit d0aa7a70bf03b9de9e995ab272293be1f7937822.

It not only introduced user space visible changes to the futex syscall,
it is also non-functional and there is no way to fix it proper before
the 2.6.22 release.

The breakage report ( http://lkml.org/lkml/2007/5/12/17 ) went
unanswered, and unfortunately it turned out that the concept is not
feasible at all.  It violates the rtmutex semantics badly by introducing
a virtual owner, which hacks around the coupling of the user-space
pi_futex and the kernel internal rt_mutex representation.

At the moment the only safe option is to remove it fully as it contains
user-space visible changes to broken kernel code, which we do not want
to expose in the 2.6.22 release.

The patch reverts the original patch mostly 1:1, but contains a couple
of trivial manual cleanups which were necessary due to patches, which
touched the same area of code later.

Verified against the glibc tests and my own PI futex tests.

Signed-off-by: Thomas Gleixner 
Acked-by: Ingo Molnar 
Acked-by: Ulrich Drepper 
Cc: Pierre Peiffer 
Signed-off-by: Linus Torvalds

FUTEX: new PRIVATE futexes

2007-05-09T19:30:55+00:00

  Analysis of current linux futex code :
  --------------------------------------

A central hash table futex_queues[] holds all contexts (futex_q) of waiting
threads.

Each futex_wait()/futex_wait() has to obtain a spinlock on a hash slot to
perform lookups or insert/deletion of a futex_q.

When a futex_wait() is done, calling thread has to :

1) - Obtain a read lock on mmap_sem to be able to validate the user pointer
     (calling find_vma()). This validation tells us if the futex uses
     an inode based store (mapped file), or mm based store (anonymous mem)

2) - compute a hash key

3) - Atomic increment of reference counter on an inode or a mm_struct

4) - lock part of futex_queues[] hash table

5) - perform the test on value of futex.
	(rollback is value != expected_value, returns EWOULDBLOCK)
	(various loops if test triggers mm faults)

6) queue the context into hash table, release the lock got in 4)

7) - release the read_lock on mmap_sem

   

8) Eventually unqueue the context (but rarely, as this part  may be done
   by the futex_wake())

Futexes were designed to improve scalability but current implementation has
various problems :

- Central hashtable :

  This means scalability problems if many processes/threads want to use
  futexes at the same time.
  This means NUMA unbalance because this hashtable is located on one node.

- Using mmap_sem on every futex() syscall :

  Even if mmap_sem is a rw_semaphore, up_read()/down_read() are doing atomic
  ops on mmap_sem, dirtying cache line :
    - lot of cache line ping pongs on SMP configurations.

  mmap_sem is also extensively used by mm code (page faults, mmap()/munmap())
  Highly threaded processes might suffer from mmap_sem contention.

  mmap_sem is also used by oprofile code. Enabling oprofile hurts threaded
  programs because of contention on the mmap_sem cache line.

- Using an atomic_inc()/atomic_dec() on inode ref counter or mm ref counter:
  It's also a cache line ping pong on SMP. It also increases mmap_sem hold time
  because of cache misses.

Most of these scalability problems come from the fact that futexes are in
one global namespace.  As we use a central hash table, we must make sure
they are all using the same reference (given by the mm subsystem).  We
chose to force all futexes be 'shared'.  This has a cost.

But fact is POSIX defined PRIVATE and SHARED, allowing clear separation,
and optimal performance if carefuly implemented.  Time has come for linux
to have better threading performance.

The goal is to permit new futex commands to avoid :
 - Taking the mmap_sem semaphore, conflicting with other subsystems.
 - Modifying a ref_count on mm or an inode, still conflicting with mm or fs.

This is possible because, for one process using PTHREAD_PROCESS_PRIVATE
futexes, we only need to distinguish futexes by their virtual address, no
matter the underlying mm storage is.

If glibc wants to exploit this new infrastructure, it should use new
_PRIVATE futex subcommands for PTHREAD_PROCESS_PRIVATE futexes.  And be
prepared to fallback on old subcommands for old kernels.  Using one global
variable with the FUTEX_PRIVATE_FLAG or 0 value should be OK.

PTHREAD_PROCESS_SHARED futexes should still use the old subcommands.

Compatibility with old applications is preserved, they still hit the
scalability problems, but new applications can fly :)

Note : the same SHARED futex (mapped on a file) can be used by old binaries
*and* new binaries, because both binaries will use the old subcommands.

Note : Vast majority of futexes should be using PROCESS_PRIVATE semantic,
as this is the default semantic. Almost all applications should benefit
of this changes (new kernel and updated libc)

Some bench results on a Pentium M 1.6 GHz (SMP kernel on a UP machine)

/* calling futex_wait(addr, value) with value != *addr */
433 cycles per futex(FUTEX_WAIT) call (mixing 2 futexes)
424 cycles per futex(FUTEX_WAIT) call (using one futex)
334 cycles per futex(FUTEX_WAIT_PRIVATE) call (mixing 2 futexes)
334 cycles per futex(FUTEX_WAIT_PRIVATE) call (using one futex)
For reference :
187 cycles per getppid() call
188 cycles per umask() call
181 cycles per ni_syscall() call

Signed-off-by: Eric Dumazet 
Pierre Peiffer 
Cc: "Ulrich Drepper" 
Cc: "Nick Piggin" 
Cc: "Ingo Molnar" 
Cc: Rusty Russell 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

futex_requeue_pi optimization

2007-05-09T19:30:55+00:00

This patch provides the futex_requeue_pi functionality, which allows some
threads waiting on a normal futex to be requeued on the wait-queue of a
PI-futex.

This provides an optimization, already used for (normal) futexes, to be used
with the PI-futexes.

This optimization is currently used by the glibc in pthread_broadcast, when
using "normal" mutexes.  With futex_requeue_pi, it can be used with
PRIO_INHERIT mutexes too.

Signed-off-by: Pierre Peiffer 
Cc: Ingo Molnar 
Cc: Ulrich Drepper 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

Make futex_wait() use an hrtimer for timeout

2007-05-09T19:30:55+00:00

This patch modifies futex_wait() to use an hrtimer + schedule() in place of
schedule_timeout().

schedule_timeout() is tick based, therefore the timeout granularity is the
tick (1 ms, 4 ms or 10 ms depending on HZ).  By using a high resolution timer
for timeout wakeup, we can attain a much finer timeout granularity (in the
microsecond range).  This parallels what is already done for futex_lock_pi().

The timeout passed to the syscall is no longer converted to jiffies and is
therefore passed to do_futex() and futex_wait() as an absolute ktime_t
therefore keeping nanosecond resolution.

Also this removes the need to pass the nanoseconds timeout part to
futex_lock_pi() in val2.

In futex_wait(), if there is no timeout then a regular schedule() is
performed.  Otherwise, an hrtimer is fired before schedule() is called.

[akpm@linux-foundation.org: fix `make headers_check']
Signed-off-by: Sebastien Dugue 
Signed-off-by: Pierre Peiffer 
Cc: Ingo Molnar 
Cc: Ulrich Drepper 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

futex: get_futex_key, get_key_refs and drop_key_refs

2007-05-08T18:15:03+00:00

lguest uses the convenient futex infrastructure for inter-domain I/O, so
expose get_futex_key, get_key_refs (renamed get_futex_key_refs) and
drop_key_refs (renamed drop_futex_key_refs).  Also means we need to expose the
union that these use.

No code changes.

Signed-off-by: Rusty Russell 
Cc: Andi Kleen 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

[PATCH] Fix noise in futex.h

2006-12-10T17:55:41+00:00

There are some kernel-only bits in the middle of  which
should be removed in what we export to userspace.

Signed-off-by: David Woodhouse 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds