Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [PATCH 1/4] sched/wakeup: Strengthen current_save_and_set_rtlock_wait_state() | Date | Tue, 14 Sep 2021 00:08:30 +0200 |
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On Thu, Sep 09 2021 at 12:59, 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).
I don't think so because both the state store and the wakeup are serialized via tsk->pi_lock.
> While there, make them consistent with the IRQ usage in > set_special_state(). > > Fixes: 5f220be21418 ("sched/wakeup: Prepare for RT sleeping spin/rwlocks") > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> > --- > include/linux/sched.h | 19 +++++++++++-------- > 1 file changed, 11 insertions(+), 8 deletions(-) > > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -245,7 +245,8 @@ struct task_group; > * if (try_lock()) > * break; > * raw_spin_unlock_irq(&lock->wait_lock); > - * schedule_rtlock(); > + * if (!cond) > + * schedule_rtlock();
cond is not really relevant here.
> * raw_spin_lock_irq(&lock->wait_lock); > * set_current_state(TASK_RTLOCK_WAIT); > * } > @@ -253,22 +254,24 @@ struct task_group; > */ > #define current_save_and_set_rtlock_wait_state() \ > do { \ > - lockdep_assert_irqs_disabled(); \ > - raw_spin_lock(¤t->pi_lock); \ > + unsigned long flags; /* may shadow */ \ > + \ > + raw_spin_lock_irqsave(¤t->pi_lock, flags); \
Why? This is solely for the rtlock use case which invokes this with interrupts disabled. So why do we need that irqsave() overhead here?
> current->saved_state = current->__state; \ > debug_rtlock_wait_set_state(); \ > - WRITE_ONCE(current->__state, TASK_RTLOCK_WAIT); \ > - raw_spin_unlock(¤t->pi_lock); \ > + smp_store_mb(current->__state, TASK_RTLOCK_WAIT); \
The try_lock() does not matter at all here, really. All what matters is that the unlocker cannot observe the wrong state and that's fully serialized via tsk::pi_lock.
Thanks,
tglx
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