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

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Message-ID: <1429386098.7346.260.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Sat, 18 Apr 2015 12:41:38 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Mateusz Guzik <mguzik@...hat.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, 2015-04-18 at 00:02 +0100, Al Viro wrote:
> 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...

I would rather use an xchg() instead of rcu_assign_ponter()

old = xchg(&fdt->fd[fd], file);
if (unlikely(old))
	filp_close(old, files);

If threads are using close() on random fds, final result is not
guaranteed anyway.


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