linux-kernel - Re: epoll: possible bug from wakeup

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Date:	Thu, 7 Mar 2013 20:56:49 -0800
From:	Arve Hjønnevåg <arve@...roid.com>
To:	Eric Wong <normalperson@...t.net>
Cc:	NeilBrown <neilb@...e.de>, "Rafael J. Wysocki" <rjw@...k.pl>,
	linux-kernel@...r.kernel.org,
	Davide Libenzi <davidel@...ilserver.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: epoll: possible bug from wakeup_source activation

On Thu, Mar 7, 2013 at 5:30 PM, Eric Wong <normalperson@...t.net> wrote:
> Eric Wong <normalperson@...t.net> wrote:
>> Hi Arve, looking at commit 4d7e30d98939a0340022ccd49325a3d70f7e0238
>> (epoll: Add a flag, EPOLLWAKEUP, to prevent suspend ...)
>>
>> I think the reason for using ep->ws instead of epi->ws in the unlikely
>> ovflist case applies to the likely rdllist case, too.  Since epi->ws is
>> only protected by ep->mtx, it can also be deactivated while inside
>> ep_poll_callback.
>>
>> So something like the following patch might be necessary
>> (shown here with extra context):
>>
>> --- a/fs/eventpoll.c
>> +++ b/fs/eventpoll.c
>> @@ -968,39 +968,45 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *k
>>       if (unlikely(ep->ovflist != EP_UNACTIVE_PTR)) {
>>               if (epi->next == EP_UNACTIVE_PTR) {
>>                       epi->next = ep->ovflist;
>>                       ep->ovflist = epi;
>>                       if (epi->ws) {
>>                               /*
>>                                * Activate ep->ws since epi->ws may get
>>                                * deactivated at any time.
>>                                */
>>                               __pm_stay_awake(ep->ws);
>>                       }
>>
>>               }
>
> Thinking about this more, it looks like the original ep->ovflist case of
> using ep->ws is unnecessary.
>
> ep->ovflist != EP_UNACTIVE_PTR can only happen while ep->mtx is held (in
> ep_scan_ready_list); which means ep_modify+friends cannot remove epi->ws.
>

The callback function in ep_scan_ready_list can call __pm_relax on it though.

> ep_poll_callback holding ep->lock means ep_poll_callback prevents
> ep_scan_ready_list from setting ep->ovflist = EP_UNACTIVE_PTR and
> releasing ep->mtx.

This code is reached when ep_scan_ready_list has set ep->ovflist to
NULL before releasing ep->lock. Since the callback function can call
__pm_relax on epi->ws without holding ep->lock we call __pm_stay_awake
in ep->ws here (the callback does not call __pm_relax on that).

>
>>               goto out_unlock;
>>       }
>>
>>       /* If this file is already in the ready list we exit soon */
>>       if (!ep_is_linked(&epi->rdllink)) {
>>               list_add_tail(&epi->rdllink, &ep->rdllist);
>> -             __pm_stay_awake(epi->ws);
>> +             if (epi->ws) {
>> +                     /*
>> +                      * Activate ep->ws since epi->ws may get
>> +                      * deactivated at any time.
>> +                      */
>> +                     __pm_stay_awake(ep->ws);
>> +             }
>>       }
>
> I still think ep->ws needs to be used in the common ep->rdllist case.

ep_scan_ready_list calls __pm_relax on ep->ws when it is done, so this
will not work. ep->ws is not a "ep->rdllist not empty wakeup_source is
is a "ep_scan_ready_list is running" wakeup_source.

-- 
Arve Hjønnevåg
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