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 00:52:29 +0200 |
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On Tue, Sep 14 2021 at 00:08, Thomas Gleixner wrote:
> 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.
If your reasoning would be correct, then set_special_state() would be broken as well.
Thanks,
tglx
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