| Message ID | 20190506074553.21464-1-daniel.vetter@ffwll.ch (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | RFC: console: hack up console_lock more v2 | expand |
On Mon 2019-05-06 09:45:53, Daniel Vetter wrote: > console_trylock, called from within printk, can be called from pretty > much anywhere. Including try_to_wake_up. Note that this isn't common, > usually the box is in pretty bad shape at that point already. But it > really doesn't help when then lockdep jumps in and spams the logs, > potentially obscuring the real backtrace we're really interested in. > One case I've seen (slightly simplified backtrace): > > Call Trace: > <IRQ> > console_trylock+0xe/0x60 > vprintk_emit+0xf1/0x320 > printk+0x4d/0x69 > __warn_printk+0x46/0x90 > native_smp_send_reschedule+0x2f/0x40 > check_preempt_curr+0x81/0xa0 > ttwu_do_wakeup+0x14/0x220 > try_to_wake_up+0x218/0x5f0 try_to_wake_up() takes p->pi_lock. It could deadlock because it can get called recursively from printk_safe_up(). And there are more locks taken from try_to_wake_up(), for example, __task_rq_lock() taken from ttwu_remote(). IMHO, the most reliable solution would be do call the entire up_console_sem() from printk deferred context. We could assign few bytes for this context in the per-CPU printk_deferred variable. Best Regards, Petr
On Mon 2019-05-06 09:45:53, Daniel Vetter wrote: > console_trylock, called from within printk, can be called from pretty > much anywhere. Including try_to_wake_up. Note that this isn't common, > usually the box is in pretty bad shape at that point already. But it > really doesn't help when then lockdep jumps in and spams the logs, > potentially obscuring the real backtrace we're really interested in. > One case I've seen (slightly simplified backtrace): > > Call Trace: > <IRQ> > console_trylock+0xe/0x60 > vprintk_emit+0xf1/0x320 > printk+0x4d/0x69 > __warn_printk+0x46/0x90 > native_smp_send_reschedule+0x2f/0x40 > check_preempt_curr+0x81/0xa0 > ttwu_do_wakeup+0x14/0x220 > try_to_wake_up+0x218/0x5f0 try_to_wake_up() takes p->pi_lock. It could deadlock because it can get called recursively from printk_safe_up(). And there are more locks taken from try_to_wake_up(), for example, __task_rq_lock() taken from ttwu_remote(). IMHO, the most reliable solution would be do call the entire up_console_sem() from printk deferred context. We could assign few bytes for this context in the per-CPU printk_deferred variable. Best Regards, Petr
On Mon 2019-05-06 10:16:14, Petr Mladek wrote: > On Mon 2019-05-06 09:45:53, Daniel Vetter wrote: > > console_trylock, called from within printk, can be called from pretty > > much anywhere. Including try_to_wake_up. Note that this isn't common, > > usually the box is in pretty bad shape at that point already. But it > > really doesn't help when then lockdep jumps in and spams the logs, > > potentially obscuring the real backtrace we're really interested in. > > One case I've seen (slightly simplified backtrace): > > > > Call Trace: > > <IRQ> > > console_trylock+0xe/0x60 > > vprintk_emit+0xf1/0x320 > > printk+0x4d/0x69 > > __warn_printk+0x46/0x90 > > native_smp_send_reschedule+0x2f/0x40 > > check_preempt_curr+0x81/0xa0 > > ttwu_do_wakeup+0x14/0x220 > > try_to_wake_up+0x218/0x5f0 > > try_to_wake_up() takes p->pi_lock. It could deadlock because it > can get called recursively from printk_safe_up(). > > And there are more locks taken from try_to_wake_up(), for example, > __task_rq_lock() taken from ttwu_remote(). > > IMHO, the most reliable solution would be do call the entire > up_console_sem() from printk deferred context. We could assign > few bytes for this context in the per-CPU printk_deferred > variable. Ah, I was too fast and did the same mistake. This won't help because it would still call try_to_wake_up() recursively. We need to call all printk's that can be called under locks taken in try_to_wake_up() path in printk deferred context. Unfortunately it is whack a mole approach. Best Regards, Petr
On Mon, May 06, 2019 at 10:26:28AM +0200, Petr Mladek wrote: > On Mon 2019-05-06 10:16:14, Petr Mladek wrote: > > On Mon 2019-05-06 09:45:53, Daniel Vetter wrote: > > > console_trylock, called from within printk, can be called from pretty > > > much anywhere. Including try_to_wake_up. Note that this isn't common, > > > usually the box is in pretty bad shape at that point already. But it > > > really doesn't help when then lockdep jumps in and spams the logs, > > > potentially obscuring the real backtrace we're really interested in. > > > One case I've seen (slightly simplified backtrace): > > > > > > Call Trace: > > > <IRQ> > > > console_trylock+0xe/0x60 > > > vprintk_emit+0xf1/0x320 > > > printk+0x4d/0x69 > > > __warn_printk+0x46/0x90 > > > native_smp_send_reschedule+0x2f/0x40 > > > check_preempt_curr+0x81/0xa0 > > > ttwu_do_wakeup+0x14/0x220 > > > try_to_wake_up+0x218/0x5f0 > > > > try_to_wake_up() takes p->pi_lock. It could deadlock because it > > can get called recursively from printk_safe_up(). > > > > And there are more locks taken from try_to_wake_up(), for example, > > __task_rq_lock() taken from ttwu_remote(). > > > > IMHO, the most reliable solution would be do call the entire > > up_console_sem() from printk deferred context. We could assign > > few bytes for this context in the per-CPU printk_deferred > > variable. > > Ah, I was too fast and did the same mistake. This won't help because > it would still call try_to_wake_up() recursively. Uh :-/ > We need to call all printk's that can be called under locks > taken in try_to_wake_up() path in printk deferred context. > Unfortunately it is whack a mole approach. Hm since it's whack-a-mole anyway, what about converting the WARN_ON into a prinkt_deferred, like all the other scheduler related code? Feels a notch more consistent to me than leaking the printk_context into areas it wasn't really meant built for. Scheduler code already fully subscribed to the whack-a-mole approach after all. This would mean we drop the backtrace from the native_smp_send_reschedule, or maybe we need a printk_deferred_backtrace()? -Daniel
On Mon 2019-05-06 11:38:13, Daniel Vetter wrote: > On Mon, May 06, 2019 at 10:26:28AM +0200, Petr Mladek wrote: > > On Mon 2019-05-06 10:16:14, Petr Mladek wrote: > > > On Mon 2019-05-06 09:45:53, Daniel Vetter wrote: > > > > console_trylock, called from within printk, can be called from pretty > > > > much anywhere. Including try_to_wake_up. Note that this isn't common, > > > > usually the box is in pretty bad shape at that point already. But it > > > > really doesn't help when then lockdep jumps in and spams the logs, > > > > potentially obscuring the real backtrace we're really interested in. > > > > One case I've seen (slightly simplified backtrace): > > > > > > > > Call Trace: > > > > <IRQ> > > > > console_trylock+0xe/0x60 > > > > vprintk_emit+0xf1/0x320 > > > > printk+0x4d/0x69 > > > > __warn_printk+0x46/0x90 > > > > native_smp_send_reschedule+0x2f/0x40 > > > > check_preempt_curr+0x81/0xa0 > > > > ttwu_do_wakeup+0x14/0x220 > > > > try_to_wake_up+0x218/0x5f0 > > > > > > try_to_wake_up() takes p->pi_lock. It could deadlock because it > > > can get called recursively from printk_safe_up(). > > > > > > And there are more locks taken from try_to_wake_up(), for example, > > > __task_rq_lock() taken from ttwu_remote(). > > > > > > IMHO, the most reliable solution would be do call the entire > > > up_console_sem() from printk deferred context. We could assign > > > few bytes for this context in the per-CPU printk_deferred > > > variable. > > > > Ah, I was too fast and did the same mistake. This won't help because > > it would still call try_to_wake_up() recursively. > > Uh :-/ > > > We need to call all printk's that can be called under locks > > taken in try_to_wake_up() path in printk deferred context. > > Unfortunately it is whack a mole approach. > > Hm since it's whack-a-mole anyway, what about converting the WARN_ON into > a prinkt_deferred, like all the other scheduler related code? Feels a > notch more consistent to me than leaking the printk_context into areas it > wasn't really meant built for. Scheduler code already fully subscribed to > the whack-a-mole approach after all. I am not sure how exactly you mean the conversion. Anyway, we do not want to use printk_deferred() treewide. It reduces the chance that the messages reach consoles. Scheduler is an exception because of the possible deadlocks. A solution would be to define WARN_ON_DEFERRED() that would call normal WARN_ON() in printk deferred context and use in scheduler. Best Regards, Petr
On Mon, May 06, 2019 at 01:24:48PM +0200, Petr Mladek wrote: > On Mon 2019-05-06 11:38:13, Daniel Vetter wrote: > > On Mon, May 06, 2019 at 10:26:28AM +0200, Petr Mladek wrote: > > > On Mon 2019-05-06 10:16:14, Petr Mladek wrote: > > > > On Mon 2019-05-06 09:45:53, Daniel Vetter wrote: > > > > > console_trylock, called from within printk, can be called from pretty > > > > > much anywhere. Including try_to_wake_up. Note that this isn't common, > > > > > usually the box is in pretty bad shape at that point already. But it > > > > > really doesn't help when then lockdep jumps in and spams the logs, > > > > > potentially obscuring the real backtrace we're really interested in. > > > > > One case I've seen (slightly simplified backtrace): > > > > > > > > > > Call Trace: > > > > > <IRQ> > > > > > console_trylock+0xe/0x60 > > > > > vprintk_emit+0xf1/0x320 > > > > > printk+0x4d/0x69 > > > > > __warn_printk+0x46/0x90 > > > > > native_smp_send_reschedule+0x2f/0x40 > > > > > check_preempt_curr+0x81/0xa0 > > > > > ttwu_do_wakeup+0x14/0x220 > > > > > try_to_wake_up+0x218/0x5f0 > > > > > > > > try_to_wake_up() takes p->pi_lock. It could deadlock because it > > > > can get called recursively from printk_safe_up(). > > > > > > > > And there are more locks taken from try_to_wake_up(), for example, > > > > __task_rq_lock() taken from ttwu_remote(). > > > > > > > > IMHO, the most reliable solution would be do call the entire > > > > up_console_sem() from printk deferred context. We could assign > > > > few bytes for this context in the per-CPU printk_deferred > > > > variable. > > > > > > Ah, I was too fast and did the same mistake. This won't help because > > > it would still call try_to_wake_up() recursively. > > > > Uh :-/ > > > > > We need to call all printk's that can be called under locks > > > taken in try_to_wake_up() path in printk deferred context. > > > Unfortunately it is whack a mole approach. > > > > Hm since it's whack-a-mole anyway, what about converting the WARN_ON into > > a prinkt_deferred, like all the other scheduler related code? Feels a > > notch more consistent to me than leaking the printk_context into areas it > > wasn't really meant built for. Scheduler code already fully subscribed to > > the whack-a-mole approach after all. > > I am not sure how exactly you mean the conversion. > > Anyway, we do not want to use printk_deferred() treewide. It reduces > the chance that the messages reach consoles. Scheduler is an > exception because of the possible deadlocks. > > A solution would be to define WARN_ON_DEFERRED() that would > call normal WARN_ON() in printk deferred context and > use in scheduler. Sent it out, and then Sergey pointed out printk_safe_enter/exit (which I guess is what you meant, and which I missed), but we're doing this already around the up() call in __up_console_sem. So I think these further recursions you're pointed out are already handled correctly, and all we need to do is to break the loop involving semaphore.lock of the console_lock semaphore only. Which I think this patch here achieves. Thoughts? Or are we again missing something here? Thanks, Daniel
Quoting Daniel Vetter (2019-05-06 08:45:53) > +/** > + * printk_safe_up - release the semaphore in console_unlock > + * @sem: the semaphore to release > + * > + * Release the semaphore. Unlike mutexes, up() may be called from any > + * context and even by tasks which have never called down(). > + * > + * NOTE: This is a special version of up() for console_unlock only. It is only > + * safe if there are no killable, interruptible or timing out down() calls. > + */ > +void printk_safe_up(struct semaphore *sem) > +{ > + unsigned long flags; > + struct semaphore_waiter *waiter = NULL; > + > + raw_spin_lock_irqsave(&sem->lock, flags); > + if (likely(list_empty(&sem->wait_list))) { > + sem->count++; > + } else { > + waiter = list_first_entry(&sem->wait_list, > + struct semaphore_waiter, list); > + list_del(&waiter->list); > + waiter->up = true; > + } > + raw_spin_unlock_irqrestore(&sem->lock, flags); > + > + if (waiter) > + wake_up_process(waiter->task); From comparing against __down_common() there's a risk here that as soon as waiter->up == true, the waiter may complete and make the onstack struct semaphore_waiter invalid. If you store waiter->task locally under the spinlock that problem is resolved. Then there is the issue of an unprotected dereference of the task in wake_up_process() -- I think you can wrap this function with rcu_read_lock() to keep that safe, and wake_up_process() should be a no-op if it races against process termination. -Chris
On Thu, May 09, 2019 at 11:32:57AM +0100, Chris Wilson wrote: > Quoting Daniel Vetter (2019-05-06 08:45:53) > > +/** > > + * printk_safe_up - release the semaphore in console_unlock > > + * @sem: the semaphore to release > > + * > > + * Release the semaphore. Unlike mutexes, up() may be called from any > > + * context and even by tasks which have never called down(). > > + * > > + * NOTE: This is a special version of up() for console_unlock only. It is only > > + * safe if there are no killable, interruptible or timing out down() calls. > > + */ > > +void printk_safe_up(struct semaphore *sem) > > +{ > > + unsigned long flags; > > + struct semaphore_waiter *waiter = NULL; > > + > > + raw_spin_lock_irqsave(&sem->lock, flags); > > + if (likely(list_empty(&sem->wait_list))) { > > + sem->count++; > > + } else { > > + waiter = list_first_entry(&sem->wait_list, > > + struct semaphore_waiter, list); > > + list_del(&waiter->list); > > + waiter->up = true; > > + } > > + raw_spin_unlock_irqrestore(&sem->lock, flags); > > + > > + if (waiter) > > + wake_up_process(waiter->task); > > From comparing against __down_common() there's a risk here that as soon > as waiter->up == true, the waiter may complete and make the onstack > struct semaphore_waiter invalid. If you store waiter->task locally under > the spinlock that problem is resolved. > > Then there is the issue of an unprotected dereference of the task in > wake_up_process() -- I think you can wrap this function with > rcu_read_lock() to keep that safe, and wake_up_process() should be a > no-op if it races against process termination. task_struct is not RCU protected, see task_rcu_dereference() for magic.
On Wed 2019-05-08 10:17:12, Daniel Vetter wrote: > On Mon, May 06, 2019 at 01:24:48PM +0200, Petr Mladek wrote: > > On Mon 2019-05-06 11:38:13, Daniel Vetter wrote: > > > On Mon, May 06, 2019 at 10:26:28AM +0200, Petr Mladek wrote: > > > > On Mon 2019-05-06 10:16:14, Petr Mladek wrote: > > > > > On Mon 2019-05-06 09:45:53, Daniel Vetter wrote: > > > > > > console_trylock, called from within printk, can be called from pretty > > > > > > much anywhere. Including try_to_wake_up. Note that this isn't common, > > > > > > usually the box is in pretty bad shape at that point already. But it > > > > > > really doesn't help when then lockdep jumps in and spams the logs, > > > > > > potentially obscuring the real backtrace we're really interested in. > > > > > > One case I've seen (slightly simplified backtrace): > > > > > > > > > > > > Call Trace: > > > > > > <IRQ> > > > > > > console_trylock+0xe/0x60 > > > > > > vprintk_emit+0xf1/0x320 > > > > > > printk+0x4d/0x69 > > > > > > __warn_printk+0x46/0x90 > > > > > > native_smp_send_reschedule+0x2f/0x40 > > > > > > check_preempt_curr+0x81/0xa0 > > > > > > ttwu_do_wakeup+0x14/0x220 > > > > > > try_to_wake_up+0x218/0x5f0 > > > > > > > > > > try_to_wake_up() takes p->pi_lock. It could deadlock because it > > > > > can get called recursively from printk_safe_up(). > > > > > > > > > > And there are more locks taken from try_to_wake_up(), for example, > > > > > __task_rq_lock() taken from ttwu_remote(). > > > > > > > > > > IMHO, the most reliable solution would be do call the entire > > > > > up_console_sem() from printk deferred context. We could assign > > > > > few bytes for this context in the per-CPU printk_deferred > > > > > variable. > > > > > > > > Ah, I was too fast and did the same mistake. This won't help because > > > > it would still call try_to_wake_up() recursively. > > > > > > Uh :-/ > > > > > > > We need to call all printk's that can be called under locks > > > > taken in try_to_wake_up() path in printk deferred context. > > > > Unfortunately it is whack a mole approach. > > > > > > Hm since it's whack-a-mole anyway, what about converting the WARN_ON into > > > a prinkt_deferred, like all the other scheduler related code? Feels a > > > notch more consistent to me than leaking the printk_context into areas it > > > wasn't really meant built for. Scheduler code already fully subscribed to > > > the whack-a-mole approach after all. > > > > I am not sure how exactly you mean the conversion. > > > > Anyway, we do not want to use printk_deferred() treewide. It reduces > > the chance that the messages reach consoles. Scheduler is an > > exception because of the possible deadlocks. > > > > A solution would be to define WARN_ON_DEFERRED() that would > > call normal WARN_ON() in printk deferred context and > > use in scheduler. > > Sent it out, and then Sergey pointed out printk_safe_enter/exit (which I > guess is what you meant, and which I missed) No, I meant introducing a deferred printk context that would behave like printk_deferred(). printk safe context is more limiting. It prevents deadlock on logbuf_lock by temporary saving the messages into per-CPU buffers. In scheduler, we could store the messages directly into the main log buffer. We just need to deffer the console handling to avoid deadlock on the scheduler locks. > , but we're doing this already around the up() call > in __up_console_sem. > > So I think these further recursions you're pointed out are already handled > correctly, and all we need to do is to break the loop involving > semaphore.lock of the console_lock semaphore only. Which I think this > patch here achieves. printk safe context would help only when try_to_wake_up() and all other functions using the same locks _all over the system_ are called printk safe (or deferred) context. By other words, printk safe context solves only printk() recursion. It does not solve recursion of the scheduler operations. Best Regards, Petr
diff --git a/include/linux/semaphore.h b/include/linux/semaphore.h index 11c86fbfeb98..7e839c72809d 100644 --- a/include/linux/semaphore.h +++ b/include/linux/semaphore.h @@ -41,6 +41,7 @@ extern int __must_check down_interruptible(struct semaphore *sem); extern int __must_check down_killable(struct semaphore *sem); extern int __must_check down_trylock(struct semaphore *sem); extern int __must_check down_timeout(struct semaphore *sem, long jiffies); +extern void printk_safe_up(struct semaphore *sem); extern void up(struct semaphore *sem); #endif /* __LINUX_SEMAPHORE_H */ diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c index 561acdd39960..84675993dc59 100644 --- a/kernel/locking/semaphore.c +++ b/kernel/locking/semaphore.c @@ -197,6 +197,37 @@ struct semaphore_waiter { bool up; }; +/** + * printk_safe_up - release the semaphore in console_unlock + * @sem: the semaphore to release + * + * Release the semaphore. Unlike mutexes, up() may be called from any + * context and even by tasks which have never called down(). + * + * NOTE: This is a special version of up() for console_unlock only. It is only + * safe if there are no killable, interruptible or timing out down() calls. + */ +void printk_safe_up(struct semaphore *sem) +{ + unsigned long flags; + struct semaphore_waiter *waiter = NULL; + + raw_spin_lock_irqsave(&sem->lock, flags); + if (likely(list_empty(&sem->wait_list))) { + sem->count++; + } else { + waiter = list_first_entry(&sem->wait_list, + struct semaphore_waiter, list); + list_del(&waiter->list); + waiter->up = true; + } + raw_spin_unlock_irqrestore(&sem->lock, flags); + + if (waiter) + wake_up_process(waiter->task); +} +EXPORT_SYMBOL(printk_safe_up); + /* * Because this function is inlined, the 'state' parameter will be * constant, and thus optimised away by the compiler. Likewise the diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 02ca827b8fac..62303929afda 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -234,7 +234,7 @@ static void __up_console_sem(unsigned long ip) mutex_release(&console_lock_dep_map, 1, ip); printk_safe_enter_irqsave(flags); - up(&console_sem); + printk_safe_up(&console_sem); printk_safe_exit_irqrestore(flags); } #define up_console_sem() __up_console_sem(_RET_IP_)
console_trylock, called from within printk, can be called from pretty much anywhere. Including try_to_wake_up. Note that this isn't common, usually the box is in pretty bad shape at that point already. But it really doesn't help when then lockdep jumps in and spams the logs, potentially obscuring the real backtrace we're really interested in. One case I've seen (slightly simplified backtrace): Call Trace: <IRQ> console_trylock+0xe/0x60 vprintk_emit+0xf1/0x320 printk+0x4d/0x69 __warn_printk+0x46/0x90 native_smp_send_reschedule+0x2f/0x40 check_preempt_curr+0x81/0xa0 ttwu_do_wakeup+0x14/0x220 try_to_wake_up+0x218/0x5f0 pollwake+0x6f/0x90 credit_entropy_bits+0x204/0x310 add_interrupt_randomness+0x18f/0x210 handle_irq+0x67/0x160 do_IRQ+0x5e/0x130 common_interrupt+0xf/0xf </IRQ> This alone isn't a problem, but the spinlock in the semaphore is also still held while waking up waiters (up() -> __up() -> try_to_wake_up() callchain), which then closes the runqueue vs. semaphore.lock loop, and upsets lockdep, which issues a circular locking splat to dmesg. Worse it upsets developers, since we don't want to spam dmesg with clutter when the machine is dying already. Fix this by creating a prinkt_safe_up() which calls wake_up_process outside of the spinlock. This isn't correct in full generality, but good enough for console_lock: - console_lock doesn't use interruptible or killable or timeout down() calls, hence an up() is the only thing that can wake up a process. Hence the process can't get woken and killed and reaped while we try to wake it up too. - semaphore.c always updates the waiter list while under the spinlock, so there's no other races. Specifically another process that races with a quick console_lock/unlock while we've dropped the spinlock already won't see our own waiter. Also cc'ing John Ogness since perhaps his printk rework fixes this all properly. Signed-off-by: Daniel Vetter <daniel.vetter@intel.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Ingo Molnar <mingo@redhat.com> Cc: Will Deacon <will.deacon@arm.com> Cc: Petr Mladek <pmladek@suse.com> Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Daniel Vetter <daniel.vetter@ffwll.ch> Cc: John Ogness <john.ogness@linutronix.de> Cc: linux-kernel@vger.kernel.org --- v2: My first attempt at patching down_trylock didn't work (thanks to Petr for pointing out), now try to have a special up() that breaks the cycle. -Daniel --- include/linux/semaphore.h | 1 + kernel/locking/semaphore.c | 31 +++++++++++++++++++++++++++++++ kernel/printk/printk.c | 2 +- 3 files changed, 33 insertions(+), 1 deletion(-)