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

From: Christoph Hellwig <hch@lst.de>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Christoph Hellwig <hch@lst.de>,
	viro@zeniv.linux.org.uk, Avi Kivity <avi@scylladb.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-aio@kvack.org, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 4/4] aio: allow direct aio poll comletions for keyed wakeups
Date: Wed, 8 Aug 2018 11:57:19 +0200	[thread overview]
Message-ID: <20180808095719.GA20653@lst.de> (raw)
In-Reply-To: <20180807090441.67f7e281f502ce448de849c1@linux-foundation.org>

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);


