Re: [PATCH] poll: allow f_op->poll to sleep, take#5

[Tejun Heo <htejun@gmail.com>]

Hello,

Davide Libenzi wrote:
> Hmmm, I just noticed that the set_current_state(TASK_INTERRUPTIBLE) at the 
> beginning of the ->poll() loop has been dropped (and it makes sense since 
> now ->poll() can sleep).

Yeah, that's exactly what the ->triggered condition replaces.

> w1) WR dev->events
> w2) MB
> w3) WR triggered (1)
> w4) WMB
> w5) WR task->state (RUNNING)
> 
> Poller side:
>  
> s1) WR task->state (TASK_INTERRUPTIBLE)
> s2) MB
> s3) RD triggered
> s4) IF0 => RD task->state (if !RUNNING -> sleep)
> s5) WR triggered (0)
> s6) MB
> s7) RD dev->events
> 
> That is, an MB before w3 (triggered=1) and a set_mb(triggered,0) at 
> s5+s6. The spinlock on the queue taken before entering pollwake() is not 
> enough to guarantee the required ordering, since a LOCK is no guarantee 
> that operations before it are visible after the LOCK.
> Without the MB at w2, it could happen [w3, s5, s7, w1] that will make us 
> miss the event *and* sleep.

Yeah, it seems we'll need something which is equivalent to smp_wmb()
in try_to_wake_up().  So, the original set_mb() should have stayed
there while just adding the latter one.  Will prep yet another take of
the patch.  Thanks for the detailed analysis.

-- 
tejun