Messages in this thread | | | Date | Sat, 21 Jul 2007 18:18:14 +0400 | From | Oleg Nesterov <> | Subject | Re: [PATCH] pi-futex: set PF_EXITING without taking ->pi_lock |
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On 07/21, Ingo Molnar wrote: > > * Oleg Nesterov <oleg@tv-sign.ru> wrote: > > > It is a bit annoying that do_exit() takes ->pi_lock to set PF_EXITING. > > All we need is to synchronize with lookup_pi_state() which saw this task > > without PF_EXITING under ->pi_lock. > > > > Change do_exit() to use spin_unlock_wait(). > > > > Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru> > > Acked-by: Ingo Molnar <mingo@elte.hu>
Thanks!
> > - spin_lock_irq(&tsk->pi_lock); > > - tsk->flags |= PF_EXITING; > > - spin_unlock_irq(&tsk->pi_lock); > > + smp_mb(); > > + spin_unlock_wait(&tsk->pi_lock); > > hm, isnt spin_unlock_wait() an SMP barrier in itself?
no, only barrier() due to cpu_relax()
> (if not then it should be.)
I think you are right, I can't imagine a valid usage of spin_unlock_wait() without a barrier.
For example, from net/dccp/ccid.c
static void ccids_write_lock(void) { spin_lock(&ccids_lock); while (atomic_read(&ccids_lockct) != 0) { spin_unlock(&ccids_lock); yield(); spin_lock(&ccids_lock); } }
static inline void ccids_read_lock(void) { atomic_inc(&ccids_lockct); spin_unlock_wait(&ccids_lock); }
This looks racy, in theory atomic_inc() and spin_unlock_wait() could be re-ordered. However, in this particular case we have an "optimized" smp_mb_after_atomic_inc(), perhaps it is good that the caller can choose the "right" barrier by hand.
Oleg.
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