linux-kernel - Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in fd

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Message-ID: <20150417230252.GE889@ZenIV.linux.org.uk>
Date:	Sat, 18 Apr 2015 00:02:52 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Mateusz Guzik <mguzik@...hat.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Yann Droneaud <ydroneaud@...eya.com>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in
 fd_install

On Sat, Apr 18, 2015 at 12:16:48AM +0200, Mateusz Guzik wrote:

> I would say this makes the use of seq counter impossible. Even if we
> decided to fall back to a lock on retry, we cannot know what to do if
> the slot is reserved - it very well could be that something called
> close, and something else reserved the slot, so putting the file inside
> could be really bad. In fact we would be putting a file for which we
> don't have a reference anymore.
> 
> However, not all hope is lost and I still think we can speed things up.
> 
> A locking primitive which only locks stuff for current cpu and has
> another mode where it locks stuff for all cpus would do the trick just
> fine. I'm not a linux guy, quick search suggests 'lglock' would do what
> I want.
> 
> table reallocation is an extremely rare operation, so this should be
> fine. It would take the lock 'globally' for given table.

It would also mean percpu_alloc() for each descriptor table...
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