Messages in this thread | | | Date | Thu, 9 Sep 2021 16:27:46 +0200 | From | Peter Zijlstra <> | 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 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.
It all sorta works in the current application because most things are serialized by ->wait_lock, but given the 'normal' wait pattern I got highly suspicious of there not being a full barrier around.
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