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

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Message-ID: <1429216923.7346.211.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Thu, 16 Apr 2015 13:42:03 -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 Thu, 2015-04-16 at 19:09 +0100, Al Viro wrote:
> On Thu, Apr 16, 2015 at 02:16:31PM +0200, Mateusz Guzik wrote:
> > @@ -165,8 +165,10 @@ static int expand_fdtable(struct files_struct *files, int nr)
> >  	cur_fdt = files_fdtable(files);
> >  	if (nr >= cur_fdt->max_fds) {
> >  		/* Continue as planned */
> > +		write_seqcount_begin(&files->fdt_seqcount);
> >  		copy_fdtable(new_fdt, cur_fdt);
> >  		rcu_assign_pointer(files->fdt, new_fdt);
> > +		write_seqcount_end(&files->fdt_seqcount);
> >  		if (cur_fdt != &files->fdtab)
> >  			call_rcu(&cur_fdt->rcu, free_fdtable_rcu);
> 
> Interesting.  AFAICS, your test doesn't step anywhere near that path,
> does it?  So basically you never hit the retries during that...

Right, but then the table is almost never changed for a given process,
as we only increase it by power of two steps.

(So I scratch my initial comment, fdt_seqcount is really mostly read)



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