linux-kernel - Re: [PATCH 3/6] fuse: wait for end of IO on release

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Date:	Mon, 25 Aug 2014 19:12:40 +0400
From:	Maxim Patlasov <mpatlasov@...allels.com>
To:	Miklos Szeredi <miklos@...redi.hu>
CC:	fuse-devel <fuse-devel@...ts.sourceforge.net>,
	Anand Avati <avati@...ster.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/6] fuse: wait for end of IO on release

On 08/22/2014 06:00 PM, Miklos Szeredi wrote:
> On Thu, Aug 21, 2014 at 6:08 PM, Maxim Patlasov <MPatlasov@...allels.com> wrote:
>> There are two types of I/O activity that can be "in progress" at the time
>> of fuse_release() execution: asynchronous read-ahead and write-back. The
>> patch ensures that they are completed before fuse_release_common sends
>> FUSE_RELEASE to userspace.
>>
>> So far as fuse_release() waits for end of async I/O, its callbacks
>> (fuse_readpages_end and fuse_writepage_finish) calling fuse_file_put cannot
>> be the last holders of fuse file anymore. To emphasize the fact, the patch
>> replaces fuse_file_put with __fuse_file_put there.
> 1) spinlock around __fuse_file_put() is unnecessary,
> wake_up/wait_event will provide the necessary synchronization.
Yes, I agree.

> 2) can't we always wait for I/O and just make the actual RELEASE
> message sync or async based on the flag?
> 3) and can't we merge the fuseblk case into this as well?

I think this is doable, but the same argument that Anand suggested for 
doing sync release selectively:

> In a real world scenario you may want to perform synchronous release 
> selectively. As such performance over lots of small files is generally 
> slow because of context switches, and a synchronous release adds an 
> extra switch.

is applicable here: if an application opened a file read-only and read a 
block initiating read-ahead, it's not obvious why the app doing close(2) 
should always wait for the end of that read-ahead. For some fs it's 
desirable, for others is not. Let's fuse daemon decide which behaviour 
is preferable.

Thanks,
Maxim
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