On Thu, Mar 7, 2013 at 5:30 PM, Eric Wong <normalperson@xxxxxxxx> wrote:
> Eric Wong <normalperson@xxxxxxxx> 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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