From 9d71b344f86f4264a5fae43c997a630e93c0de9b Mon Sep 17 00:00:00 2001 From: Sumit Garg Date: Thu, 4 Jun 2020 15:31:16 +0530 Subject: kdb: Re-factor kdb_printf() message write code Re-factor kdb_printf() message write code in order to avoid duplication of code and thereby increase readability. Signed-off-by: Sumit Garg Reviewed-by: Douglas Anderson Reviewed-by: Petr Mladek Link: https://lore.kernel.org/r/1591264879-25920-2-git-send-email-sumit.garg@linaro.org Signed-off-by: Daniel Thompson --- kernel/debug/kdb/kdb_io.c | 57 +++++++++++++++++++++++------------------------ 1 file changed, 28 insertions(+), 29 deletions(-) (limited to 'kernel/debug') diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c index 924bc9298a42..2d42a02de40e 100644 --- a/kernel/debug/kdb/kdb_io.c +++ b/kernel/debug/kdb/kdb_io.c @@ -542,6 +542,29 @@ static int kdb_search_string(char *searched, char *searchfor) return 0; } +static void kdb_msg_write(const char *msg, int msg_len) +{ + struct console *c; + + if (msg_len == 0) + return; + + if (dbg_io_ops && !dbg_io_ops->is_console) { + const char *cp = msg; + int len = msg_len; + + while (len--) { + dbg_io_ops->write_char(*cp); + cp++; + } + } + + for_each_console(c) { + c->write(c, msg, msg_len); + touch_nmi_watchdog(); + } +} + int vkdb_printf(enum kdb_msgsrc src, const char *fmt, va_list ap) { int diag; @@ -553,7 +576,6 @@ int vkdb_printf(enum kdb_msgsrc src, const char *fmt, va_list ap) int this_cpu, old_cpu; char *cp, *cp2, *cphold = NULL, replaced_byte = ' '; char *moreprompt = "more> "; - struct console *c; unsigned long uninitialized_var(flags); /* Serialize kdb_printf if multiple cpus try to write at once. @@ -687,22 +709,11 @@ kdb_printit: */ retlen = strlen(kdb_buffer); cp = (char *) printk_skip_headers(kdb_buffer); - if (!dbg_kdb_mode && kgdb_connected) { + if (!dbg_kdb_mode && kgdb_connected) gdbstub_msg_write(cp, retlen - (cp - kdb_buffer)); - } else { - if (dbg_io_ops && !dbg_io_ops->is_console) { - len = retlen - (cp - kdb_buffer); - cp2 = cp; - while (len--) { - dbg_io_ops->write_char(*cp2); - cp2++; - } - } - for_each_console(c) { - c->write(c, cp, retlen - (cp - kdb_buffer)); - touch_nmi_watchdog(); - } - } + else + kdb_msg_write(cp, retlen - (cp - kdb_buffer)); + if (logging) { saved_loglevel = console_loglevel; console_loglevel = CONSOLE_LOGLEVEL_SILENT; @@ -751,19 +762,7 @@ kdb_printit: moreprompt = "more> "; kdb_input_flush(); - - if (dbg_io_ops && !dbg_io_ops->is_console) { - len = strlen(moreprompt); - cp = moreprompt; - while (len--) { - dbg_io_ops->write_char(*cp); - cp++; - } - } - for_each_console(c) { - c->write(c, moreprompt, strlen(moreprompt)); - touch_nmi_watchdog(); - } + kdb_msg_write(moreprompt, strlen(moreprompt)); if (logging) printk("%s", moreprompt); -- cgit v1.2.3 From e8857288bb620d594c94a219148d18562e52b06e Mon Sep 17 00:00:00 2001 From: Sumit Garg Date: Thu, 4 Jun 2020 15:31:17 +0530 Subject: kdb: Check status of console prior to invoking handlers Check if a console is enabled prior to invoking corresponding write handler. Suggested-by: Sergey Senozhatsky Signed-off-by: Sumit Garg Reviewed-by: Daniel Thompson Reviewed-by: Douglas Anderson Reviewed-by: Petr Mladek Link: https://lore.kernel.org/r/1591264879-25920-3-git-send-email-sumit.garg@linaro.org Signed-off-by: Daniel Thompson --- kernel/debug/kdb/kdb_io.c | 2 ++ 1 file changed, 2 insertions(+) (limited to 'kernel/debug') diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c index 2d42a02de40e..58b7d256d0a4 100644 --- a/kernel/debug/kdb/kdb_io.c +++ b/kernel/debug/kdb/kdb_io.c @@ -560,6 +560,8 @@ static void kdb_msg_write(const char *msg, int msg_len) } for_each_console(c) { + if (!(c->flags & CON_ENABLED)) + continue; c->write(c, msg, msg_len); touch_nmi_watchdog(); } -- cgit v1.2.3 From 2a78b85b70f9c3d450619d369d349ba861320510 Mon Sep 17 00:00:00 2001 From: Sumit Garg Date: Thu, 4 Jun 2020 15:31:18 +0530 Subject: kdb: Make kdb_printf() console handling more robust While rounding up CPUs via NMIs, its possible that a rounded up CPU maybe holding a console port lock leading to kgdb master CPU stuck in a deadlock during invocation of console write operations. A similar deadlock could also be possible while using synchronous breakpoints. So in order to avoid such a deadlock, set oops_in_progress to encourage the console drivers to disregard their internal spin locks: in the current calling context the risk of deadlock is a bigger problem than risks due to re-entering the console driver. We operate directly on oops_in_progress rather than using bust_spinlocks() because the calls bust_spinlocks() makes on exit are not appropriate for this calling context. Suggested-by: Sergey Senozhatsky Signed-off-by: Sumit Garg Reviewed-by: Douglas Anderson Reviewed-by: Petr Mladek Link: https://lore.kernel.org/r/1591264879-25920-4-git-send-email-sumit.garg@linaro.org Signed-off-by: Daniel Thompson --- kernel/debug/kdb/kdb_io.c | 11 +++++++++++ 1 file changed, 11 insertions(+) (limited to 'kernel/debug') diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c index 58b7d256d0a4..0e4f2eda96d8 100644 --- a/kernel/debug/kdb/kdb_io.c +++ b/kernel/debug/kdb/kdb_io.c @@ -562,7 +562,18 @@ static void kdb_msg_write(const char *msg, int msg_len) for_each_console(c) { if (!(c->flags & CON_ENABLED)) continue; + /* + * Set oops_in_progress to encourage the console drivers to + * disregard their internal spin locks: in the current calling + * context the risk of deadlock is a bigger problem than risks + * due to re-entering the console driver. We operate directly on + * oops_in_progress rather than using bust_spinlocks() because + * the calls bust_spinlocks() makes on exit are not appropriate + * for this calling context. + */ + ++oops_in_progress; c->write(c, msg, msg_len); + --oops_in_progress; touch_nmi_watchdog(); } } -- cgit v1.2.3 From 5946d1f5b309381805bad3ddc3054c04f4ae9c24 Mon Sep 17 00:00:00 2001 From: Sumit Garg Date: Thu, 4 Jun 2020 15:31:19 +0530 Subject: kdb: Switch to use safer dbg_io_ops over console APIs In kgdb context, calling console handlers aren't safe due to locks used in those handlers which could in turn lead to a deadlock. Although, using oops_in_progress increases the chance to bypass locks in most console handlers but it might not be sufficient enough in case a console uses more locks (VT/TTY is good example). Currently when a driver provides both polling I/O and a console then kdb will output using the console. We can increase robustness by using the currently active polling I/O driver (which should be lockless) instead of the corresponding console. For several common cases (e.g. an embedded system with a single serial port that is used both for console output and debugger I/O) this will result in no console handler being used. In order to achieve this we need to reverse the order of preference to use dbg_io_ops (uses polling I/O mode) over console APIs. So we just store "struct console" that represents debugger I/O in dbg_io_ops and while emitting kdb messages, skip console that matches dbg_io_ops console in order to avoid duplicate messages. After this change, "is_console" param becomes redundant and hence removed. Suggested-by: Daniel Thompson Signed-off-by: Sumit Garg Link: https://lore.kernel.org/r/1591264879-25920-5-git-send-email-sumit.garg@linaro.org Reviewed-by: Douglas Anderson Reviewed-by: Petr Mladek Acked-by: Greg Kroah-Hartman Signed-off-by: Daniel Thompson --- kernel/debug/kdb/kdb_io.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) (limited to 'kernel/debug') diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c index 0e4f2eda96d8..683a799618ad 100644 --- a/kernel/debug/kdb/kdb_io.c +++ b/kernel/debug/kdb/kdb_io.c @@ -549,7 +549,7 @@ static void kdb_msg_write(const char *msg, int msg_len) if (msg_len == 0) return; - if (dbg_io_ops && !dbg_io_ops->is_console) { + if (dbg_io_ops) { const char *cp = msg; int len = msg_len; @@ -562,6 +562,8 @@ static void kdb_msg_write(const char *msg, int msg_len) for_each_console(c) { if (!(c->flags & CON_ENABLED)) continue; + if (c == dbg_io_ops->cons) + continue; /* * Set oops_in_progress to encourage the console drivers to * disregard their internal spin locks: in the current calling -- cgit v1.2.3 From 440ab9e10e2e6e5fd677473ee6f9e3af0f6904d6 Mon Sep 17 00:00:00 2001 From: Douglas Anderson Date: Tue, 2 Jun 2020 15:47:39 -0700 Subject: kgdb: Avoid suspicious RCU usage warning At times when I'm using kgdb I see a splat on my console about suspicious RCU usage. I managed to come up with a case that could reproduce this that looked like this: WARNING: suspicious RCU usage 5.7.0-rc4+ #609 Not tainted ----------------------------- kernel/pid.c:395 find_task_by_pid_ns() needs rcu_read_lock() protection! other info that might help us debug this: rcu_scheduler_active = 2, debug_locks = 1 3 locks held by swapper/0/1: #0: ffffff81b6b8e988 (&dev->mutex){....}-{3:3}, at: __device_attach+0x40/0x13c #1: ffffffd01109e9e8 (dbg_master_lock){....}-{2:2}, at: kgdb_cpu_enter+0x20c/0x7ac #2: ffffffd01109ea90 (dbg_slave_lock){....}-{2:2}, at: kgdb_cpu_enter+0x3ec/0x7ac stack backtrace: CPU: 7 PID: 1 Comm: swapper/0 Not tainted 5.7.0-rc4+ #609 Hardware name: Google Cheza (rev3+) (DT) Call trace: dump_backtrace+0x0/0x1b8 show_stack+0x1c/0x24 dump_stack+0xd4/0x134 lockdep_rcu_suspicious+0xf0/0x100 find_task_by_pid_ns+0x5c/0x80 getthread+0x8c/0xb0 gdb_serial_stub+0x9d4/0xd04 kgdb_cpu_enter+0x284/0x7ac kgdb_handle_exception+0x174/0x20c kgdb_brk_fn+0x24/0x30 call_break_hook+0x6c/0x7c brk_handler+0x20/0x5c do_debug_exception+0x1c8/0x22c el1_sync_handler+0x3c/0xe4 el1_sync+0x7c/0x100 rpmh_rsc_probe+0x38/0x420 platform_drv_probe+0x94/0xb4 really_probe+0x134/0x300 driver_probe_device+0x68/0x100 __device_attach_driver+0x90/0xa8 bus_for_each_drv+0x84/0xcc __device_attach+0xb4/0x13c device_initial_probe+0x18/0x20 bus_probe_device+0x38/0x98 device_add+0x38c/0x420 If I understand properly we should just be able to blanket kgdb under one big RCU read lock and the problem should go away. We'll add it to the beast-of-a-function known as kgdb_cpu_enter(). With this I no longer get any splats and things seem to work fine. Signed-off-by: Douglas Anderson Link: https://lore.kernel.org/r/20200602154729.v2.1.I70e0d4fd46d5ed2aaf0c98a355e8e1b7a5bb7e4e@changeid Signed-off-by: Daniel Thompson --- kernel/debug/debug_core.c | 4 ++++ 1 file changed, 4 insertions(+) (limited to 'kernel/debug') diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c index bc8d25f2ac8a..9e5934780f41 100644 --- a/kernel/debug/debug_core.c +++ b/kernel/debug/debug_core.c @@ -587,6 +587,7 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs, arch_kgdb_ops.disable_hw_break(regs); acquirelock: + rcu_read_lock(); /* * Interrupts will be restored by the 'trap return' code, except when * single stepping. @@ -646,6 +647,7 @@ return_normal: atomic_dec(&slaves_in_kgdb); dbg_touch_watchdogs(); local_irq_restore(flags); + rcu_read_unlock(); return 0; } cpu_relax(); @@ -664,6 +666,7 @@ return_normal: raw_spin_unlock(&dbg_master_lock); dbg_touch_watchdogs(); local_irq_restore(flags); + rcu_read_unlock(); goto acquirelock; } @@ -787,6 +790,7 @@ kgdb_restore: raw_spin_unlock(&dbg_master_lock); dbg_touch_watchdogs(); local_irq_restore(flags); + rcu_read_unlock(); return kgdb_info[cpu].ret_state; } -- cgit v1.2.3