linux-kernel - Re: [PATCH 4/4] aio: allow direct aio poll comletions for keyed wakeups

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Date:   Wed, 8 Aug 2018 11:57:19 +0200
From:   Christoph Hellwig <hch@....de>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Christoph Hellwig <hch@....de>, viro@...iv.linux.org.uk,
        Avi Kivity <avi@...lladb.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-aio@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] aio: allow direct aio poll comletions for keyed
        wakeups

On Tue, Aug 07, 2018 at 09:04:41AM -0700, Andrew Morton wrote:
> > Because it is faster obviously.  I can update the comment.
> 
> I meant the comment could explain why it's a trylock instead of a
> spin_lock().

We could something like this the patch below.

Al, do you want me to resend or can you just fold it in?

diff --git a/fs/aio.c b/fs/aio.c
index 5943098a87c6..84df2c2bf80b 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -1684,7 +1684,8 @@ static int aio_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
 
 		/*
 		 * Try to complete the iocb inline if we can to avoid a costly
-		 * context switch.
+		 * context switch.  As the waitqueue lock nests inside the ctx
+		 * lock we can only do that if we can get it without waiting.
 		 */
 		if (spin_trylock(&iocb->ki_ctx->ctx_lock)) {
 			list_del(&iocb->ki_list);