Messages in this thread | | | Date | Fri, 10 Sep 2021 16:06:40 +0100 | From | Will Deacon <> | Subject | Re: [PATCH 1/4] sched/wakeup: Strengthen current_save_and_set_rtlock_wait_state() |
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On Fri, Sep 10, 2021 at 04:01:29PM +0200, Peter Zijlstra wrote: > On Fri, Sep 10, 2021 at 03:17:04PM +0200, Peter Zijlstra wrote: > > On Fri, Sep 10, 2021 at 01:57:26PM +0100, Will Deacon wrote: > > > On Thu, Sep 09, 2021 at 04:27:46PM +0200, Peter Zijlstra wrote: > > > > Moo yes, so the earlier changelog I wrote was something like: > > > > > > > > current_save_and_set_rtlock_wait_state(); > > > > for (;;) { > > > > if (try_lock()) > > > > break; > > > > > > > > raw_spin_unlock_irq(&lock->wait_lock); > > > > if (!cond) > > > > schedule(); > > > > raw_spin_lock_irq(&lock->wait_lock); > > > > > > > > set_current_state(TASK_RTLOCK_WAIT); > > > > } > > > > current_restore_rtlock_saved_state(); > > > > > > > > which is more what the code looks like before these patches, and in that > > > > case the @cond load can be lifted before __state. > > > > > > Ah, so that makes more sense, thanks. I can't see how the try_lock() could > > > be reordered though, as it's going to have to do an atomic rmw. > > > > OK, lemme go update the Changelog and make it __flags for bigeasy :-) > > The patch now reads: > > --- > Subject: sched/wakeup: Strengthen current_save_and_set_rtlock_wait_state() > From: Peter Zijlstra <peterz@infradead.org> > Date: Thu, 09 Sep 2021 12:59:16 +0200 > > While looking at current_save_and_set_rtlock_wait_state() I'm thinking > it really ought to use smp_store_mb(), because using it for a more > traditional wait loop like: > > current_save_and_set_rtlock_wait_state(); > for (;;) { > if (cond) > schedule(); > > set_current_state(TASK_RTLOCK_WAIT); > } > current_restore_rtlock_saved_state(); > > is actually broken, since the cond load could be re-ordered against > the state store, which could lead to a missed wakeup -> BAD (tm). > > While there, make them consistent with the IRQ usage in > set_special_state().
Cheers, that's much better:
Acked-by: Will Deacon <will@kernel.org>
Will
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