linux-kernel - Re: [PATCH 1/2] epoll: use the waitqueue lock to protect ep->wq

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Date:   Thu, 7 Dec 2017 01:49:15 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Christoph Hellwig <hch@....de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jason Baron <jbaron@...mai.com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] epoll: use the waitqueue lock to protect ep->wq


* Christoph Hellwig <hch@....de> wrote:

> The eoll code currently always uses the unlocked waitqueue helpers for

s/eoll
 /epoll

> ep->wq, but instead of holding the lock inside the waitqueue around these
> calls, as expected by the epoll code uses its own lock.

Hm, that reads a bit weirdly. How about:

 The epoll code currently uses the unlocked waitqueue helpers for managing
 ep->wq, but instead of holding the waitqueue lock around these calls, it
 uses its own ep->lock spinlock.


> Given that the
> waitqueue is not exposed to the rest of the kernel this actually works
> ok at the moment, but prevents the epoll locking rules from being
> enforced using lockdep.  Remove ep->lock and use the waitqueue lock
> to not only reduce the size of struct eventpoll but also make sure we
> can assert locking invariations in the waitqueue code.

s/but also make sure
  but also to make sure

s/invariations
 /invariants

> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
>  fs/eventpoll.c | 46 ++++++++++++++++++++++------------------------
>  1 file changed, 22 insertions(+), 24 deletions(-)
> 
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index afd548ebc328..2b2c5ac80e26 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -182,11 +182,10 @@ struct epitem {
>   * This structure is stored inside the "private_data" member of the file
>   * structure and represents the main data structure for the eventpoll
>   * interface.
> + *
> + * Access to it is protected by the lock inside wq.
>   */
>  struct eventpoll {
> -	/* Protect the access to this structure */
> -	spinlock_t lock;
> -
>  	/*
>  	 * This mutex is used to ensure that files are not removed
>  	 * while epoll is using them. This is held during the event
> @@ -210,7 +209,7 @@ struct eventpoll {
>  	/*
>  	 * This is a single linked list that chains all the "struct epitem" that
>  	 * happened while transferring ready events to userspace w/out
> -	 * holding ->lock.
> +	 * holding ->wq.lock.
>  	 */

Neat trick!

This exposes some waitqueue internals, but AFAICS the FUSE code already does a 
similar trick with fiq->waitq.lock so there's precedent.

Peter, what do you think?

Thanks,

	Ingo