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 _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/intel-gfx
