On 07/21, Ingo Molnar wrote:
>
> * Oleg Nesterov <[EMAIL PROTECTED]> 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 <[EMAIL PROTECTED]>
>
> Acked-by: Ingo Molnar <[EMAIL PROTECTED]>
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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