Messages in this thread | | | Date | Tue, 24 Sep 2019 19:52:18 +0200 | From | Roman Penyaev <> | Subject | Re: [PATCH] epoll: simplify ep_poll_safewake() for CONFIG_DEBUG_LOCK_ALLOC |
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On 2019-09-24 19:34, Jason Baron wrote: > On 9/23/19 3:23 PM, Roman Penyaev wrote: >> On 2019-09-23 17:43, Jason Baron wrote: >>> On 9/4/19 4:22 PM, Jason Baron wrote: >>>> Currently, ep_poll_safewake() in the CONFIG_DEBUG_LOCK_ALLOC case >>>> uses >>>> ep_call_nested() in order to pass the correct subclass argument to >>>> spin_lock_irqsave_nested(). However, ep_call_nested() adds >>>> unnecessary >>>> checks for epoll depth and loops that are already verified when >>>> doing >>>> EPOLL_CTL_ADD. This mirrors a conversion that was done for >>>> !CONFIG_DEBUG_LOCK_ALLOC in: commit 37b5e5212a44 ("epoll: remove >>>> ep_call_nested() from ep_eventpoll_poll()") >>>> >>>> Signed-off-by: Jason Baron <jbaron@akamai.com> >>>> Cc: Davidlohr Bueso <dave@stgolabs.net> >>>> Cc: Roman Penyaev <rpenyaev@suse.de> >>>> Cc: Al Viro <viro@zeniv.linux.org.uk> >>>> Cc: Eric Wong <normalperson@yhbt.net> >>>> Cc: Andrew Morton <akpm@linux-foundation.org> >>>> --- >>>> fs/eventpoll.c | 36 +++++++++++++----------------------- >>>> 1 file changed, 13 insertions(+), 23 deletions(-) >>>> >>>> diff --git a/fs/eventpoll.c b/fs/eventpoll.c >>>> index d7f1f50..a9b2737 100644 >>>> --- a/fs/eventpoll.c >>>> +++ b/fs/eventpoll.c >>>> @@ -551,28 +551,23 @@ static int ep_call_nested(struct nested_calls >>>> *ncalls, >>>> */ >>>> #ifdef CONFIG_DEBUG_LOCK_ALLOC >>>> >>>> -static struct nested_calls poll_safewake_ncalls; >>>> - >>>> -static int ep_poll_wakeup_proc(void *priv, void *cookie, int >>>> call_nests) >>>> -{ >>>> - unsigned long flags; >>>> - wait_queue_head_t *wqueue = (wait_queue_head_t *)cookie; >>>> - >>>> - spin_lock_irqsave_nested(&wqueue->lock, flags, call_nests + 1); >>>> - wake_up_locked_poll(wqueue, EPOLLIN); >>>> - spin_unlock_irqrestore(&wqueue->lock, flags); >>>> - >>>> - return 0; >>>> -} >>>> +static DEFINE_PER_CPU(int, wakeup_nest); >>>> >>>> static void ep_poll_safewake(wait_queue_head_t *wq) >>>> { >>>> - int this_cpu = get_cpu(); >>>> - >>>> - ep_call_nested(&poll_safewake_ncalls, >>>> - ep_poll_wakeup_proc, NULL, wq, (void *) (long) >>>> this_cpu); >>>> + unsigned long flags; >>>> + int subclass; >>>> >>>> - put_cpu(); >>>> + local_irq_save(flags); >>>> + preempt_disable(); >>>> + subclass = __this_cpu_read(wakeup_nest); >>>> + spin_lock_nested(&wq->lock, subclass + 1); >>>> + __this_cpu_inc(wakeup_nest); >>>> + wake_up_locked_poll(wq, POLLIN); >>>> + __this_cpu_dec(wakeup_nest); >>>> + spin_unlock(&wq->lock); >>>> + local_irq_restore(flags); >>>> + preempt_enable(); >>>> } >> >> What if reduce number of lines with something as the following: >> >> int this_cpu = get_cpu(); >> subclass = __this_cpu_inc_return(wakeup_nest); >> spin_lock_irqsave_nested(&wq->lock, flags, subclass); >> wake_up_locked_poll(wq, POLLIN); >> spin_unlock_irqrestore(&wq->lock, flags); >> __this_cpu_dec(wakeup_nest); >> put_cpu(); >> >> Other than that looks good to me. >> >> Reviewed-by: Roman Penyaev <rpenyaev@suse.de> >> >> -- >> Roman > > > Hi, > > I put the local_irq_save(flags), call there first so that there > wouldn't > be any nesting. For example, in your sequence, there could be an irq > after the __this_cpu_inc_return(), that could end up back here.
That is correct, but seems this is the original behavior of ep_call_nested(), where irq can happen just after spin_unlock_irqrestore():
spin_unlock_irqrestore(&ncalls->lock, flags);
>>>> irq here <<<<<
/* Call the nested function */ error = (*nproc)(priv, cookie, call_nests);
so eventually you end up with spin_lock_irqsave_nested() call where call_nests is not monotonically increased (not sequential) but has a gap (depends on nesting).
So if shorter, I thought that your "local_irq_save + increment" sequence is excessive.
-- Roman
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