linux-kernel - Re: [PATCH v2] fs/file.c: don't acquire files->file_lock in fd

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Message-ID: <1435037481.4110.4.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Tue, 23 Jun 2015 07:31:21 +0200
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: [PATCH v2] fs/file.c: don't acquire files->file_lock in
 fd_install()

On Mon, 2015-06-22 at 03:32 +0100, Al Viro wrote:
> On Tue, Apr 28, 2015 at 09:25:03PM -0700, Eric Dumazet wrote:
> 
> > @@ -553,11 +572,20 @@ void __fd_install(struct files_struct *files, unsigned int fd,
> >  		struct file *file)
> >  {
> >  	struct fdtable *fdt;
> > -	spin_lock(&files->file_lock);
> > -	fdt = files_fdtable(files);
> > +
> > +	rcu_read_lock_sched();
> > +
> > +	while (unlikely(files->resize_in_progress)) {
> > +		rcu_read_unlock_sched();
> > +		wait_event(files->resize_wait, !files->resize_in_progress);
> > +		rcu_read_lock_sched();
> > +	}
> > +	/* coupled with smp_wmb() in expand_fdtable() */
> > +	smp_rmb();
> > +	fdt = rcu_dereference_sched(files->fdt);
> >  	BUG_ON(fdt->fd[fd] != NULL);
> >  	rcu_assign_pointer(fdt->fd[fd], file);
> > -	spin_unlock(&files->file_lock);
> > +	rcu_read_unlock_sched();
> 
> Umm...  You've taken something that was safe to use in atomic contexts
> and turned into something that might wait for GFP_KERNEL allocation; what's
> to guarantee that no users get broken by that?  At the very least, you want
> to slap might_sleep() in there - the actual sleep is going to be very rare,
> so it would be an extremely hard to reproduce and debug.
> 
> AFAICS, all current in-tree users should be safe, but fd_install() is exported
> and quiet changes of that sort are rather antisocial.  Generally I don't give
> a damn about out-of-tree code, but this one is over the top.
> 
> I _think_ it's otherwise OK, but please, add might_sleep() *AND* a note in
> Documentation/filesystems/porting.
> 

Good points. I am currently traveling and will address this asap.

Thanks


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