Messages in this thread | | | Date | Fri, 10 Sep 2021 13:57:26 +0100 | From | Will Deacon <> | Subject | Re: [PATCH 1/4] sched/wakeup: Strengthen current_save_and_set_rtlock_wait_state() |
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On Thu, Sep 09, 2021 at 04:27:46PM +0200, Peter Zijlstra wrote: > On Thu, Sep 09, 2021 at 02:45:24PM +0100, Will Deacon wrote: > > On Thu, Sep 09, 2021 at 12:59:16PM +0200, Peter Zijlstra wrote: > > > While looking at current_save_and_set_rtlock_wait_state() I'm thinking > > > it really ought to use smp_store_mb(), because something like: > > > > > > current_save_and_set_rtlock_wait_state(); > > > for (;;) { > > > if (try_lock()) > > > break; > > > > > > raw_spin_unlock_irq(&lock->wait_lock); > > > schedule(); > > > raw_spin_lock_irq(&lock->wait_lock); > > > > > > set_current_state(TASK_RTLOCK_WAIT); > > > } > > > current_restore_rtlock_saved_state(); > > > > > > which is the advertised usage in the comment, is actually broken, > > > since trylock() will only need a load-acquire in general and that > > > could be re-ordered against the state store, which could lead to a > > > missed wakeup -> BAD (tm). > > > > Why doesn't the UNLOCK of pi_lock in current_save_and_set_rtlock_wait_state() > > order the state change before the successful try_lock? I'm just struggling > > to envisage how this actually goes wrong. > > 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.
Will
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