linux-kernel - Re: [PATCH] hrtimer: make epoll_wait() use the hrtimer range feature

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Message-ID: <AANLkTi=oTq3_-7OZZwGrYjQPXA0TCDFB_DaFXJ7rvWH9@mail.gmail.com>
Date:	Wed, 24 Nov 2010 15:57:13 -0500
From:	Mike Frysinger <vapier.adi@...il.com>
To:	Shawn Bohrer <shawn.bohrer@...il.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hrtimer: make epoll_wait() use the hrtimer range feature

On Wed, Nov 24, 2010 at 09:52, Shawn Bohrer wrote:
> On Wed, Nov 24, 2010 at 03:33:02AM -0500, Mike Frysinger wrote:
>> On Sun, Aug 8, 2010 at 18:45, Shawn Bohrer wrote:
>> > @@ -1116,18 +1113,22 @@ static int ep_send_events(struct eventpoll *ep,
>> >  static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
>> >                   int maxevents, long timeout)
>> >  {
>> > -       int res, eavail;
>> > +       int res, eavail, timed_out = 0;
>> >        unsigned long flags;
>> > -       long jtimeout;
>> > +       long slack;
>> >        wait_queue_t wait;
>> > -
>> > -       /*
>> > -        * Calculate the timeout by checking for the "infinite" value (-1)
>> > -        * and the overflow condition. The passed timeout is in milliseconds,
>> > -        * that why (t * HZ) / 1000.
>> > -        */
>> > -       jtimeout = (timeout < 0 || timeout >= EP_MAX_MSTIMEO) ?
>> > -               MAX_SCHEDULE_TIMEOUT : (timeout * HZ + 999) / 1000;
>> > +       struct timespec end_time;
>> > +       ktime_t expires, *to = NULL;
>> > +
>> > +       if (timeout > 0) {
>> > +               ktime_get_ts(&end_time);
>> > +               timespec_add_ns(&end_time, (u64)timeout * NSEC_PER_MSEC);
>> > +               slack = estimate_accuracy(&end_time);
>> > +               to = &expires;
>> > +               *to = timespec_to_ktime(end_time);
>> > +       } else if (timeout == 0) {
>> > +               timed_out = 1;
>> > +       }
>> >
>> >  retry:
>> >        spin_lock_irqsave(&ep->lock, flags);
>> > @@ -1149,7 +1150,7 @@ retry:
>> >                         * to TASK_INTERRUPTIBLE before doing the checks.
>> >                         */
>> >                        set_current_state(TASK_INTERRUPTIBLE);
>> > -                       if (!list_empty(&ep->rdllist) || !jtimeout)
>> > +                       if (!list_empty(&ep->rdllist) || timed_out)
>> >                                break;
>> >                        if (signal_pending(current)) {
>> >                                res = -EINTR;
>> > @@ -1157,7 +1158,9 @@ retry:
>> >                        }
>> >
>> >                        spin_unlock_irqrestore(&ep->lock, flags);
>> > -                       jtimeout = schedule_timeout(jtimeout);
>> > +                       if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS))
>> > +                               timed_out = 1;
>> > +
>> >                        spin_lock_irqsave(&ep->lock, flags);
>> >                }
>> >                __remove_wait_queue(&ep->wq, &wait);
>>
>> this code introduces a warning:
>> fs/eventpoll.c: In function ‘ep_poll’:
>> fs/eventpoll.c:1119: warning: ‘slack’ may be used uninitialized in this function
>>
>> looks to me like you arent properly handling negative timeouts.
>> certainly epoll_wait() passes the timeout value straight from
>> userspace to ep_poll() without any error checking, so if userspace
>> passes a negative timeout value, it looks like "slack" will be used
>> uninitialized.
>
> If a negative timeout is passed in then 'to' remains NULL.  When 'to
> is NULL schedule_hrtimeout_range() has an infinite timeout and the
> 'slack' parameter is never used.  So technically everything should be
> fine here.

ok, but that depends on an external function never changing behavior
and makes changing the API pretty hard since all callers must be
closely analyzed

> Of course it would be safest and best to simply initialize slack to 0.
> I can send a patch this evening with the fix.

thanks !
-mike
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