linux-kernel - Re: [PATCH] epoll: simplify ep_poll_safewake() for CONFIG_DEBUG_LOCK

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Date:   Mon, 23 Sep 2019 21:23:57 +0200
From:   Roman Penyaev <rpenyaev@...e.de>
To:     Jason Baron <jbaron@...mai.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        Davidlohr Bueso <dave@...olabs.net>,
        Al Viro <viro@...iv.linux.org.uk>,
        Eric Wong <normalperson@...t.net>
Subject: Re: [PATCH] epoll: simplify ep_poll_safewake() for
 CONFIG_DEBUG_LOCK_ALLOC

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@...mai.com>
>> Cc: Davidlohr Bueso <dave@...olabs.net>
>> Cc: Roman Penyaev <rpenyaev@...e.de>
>> Cc: Al Viro <viro@...iv.linux.org.uk>
>> Cc: Eric Wong <normalperson@...t.net>
>> Cc: Andrew Morton <akpm@...ux-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@...e.de>

--
Roman