From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Florian Weimer <fweimer@redhat.com>
Cc: NeilBrown <neilb@suse.com>,
Alexander Viro <viro@zeniv.linux.org.uk>,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
Josh Triplett <josh@joshtriplett.org>
Subject: Re: [PATCH] VFS: use synchronize_rcu_expedited() in namespace_unlock()
Date: Mon, 27 Nov 2017 06:41:25 -0800 [thread overview]
Message-ID: <20171127144125.GF3624@linux.vnet.ibm.com> (raw)
In-Reply-To: <b8a1a898-850c-cc7a-2574-1bfd15cc9888@redhat.com>
On Mon, Nov 27, 2017 at 12:27:04PM +0100, Florian Weimer wrote:
> On 10/26/2017 02:27 PM, Paul E. McKenney wrote:
> >But just for completeness, one way to make this work across the board
> >might be to instead use call_rcu(), with the callback function kicking
> >off a workqueue handler to do the rest of the unmount. Of course,
> >in saying that, I am ignoring any mutexes that you might be holding
> >across this whole thing, and also ignoring any problems that might arise
> >when returning to userspace with some portion of the unmount operation
> >still pending. (For example, someone unmounting a filesystem and then
> >immediately remounting that same filesystem.)
>
> You really need to complete all side effects of deallocating a
> resource before returning to user space. Otherwise, it will never
> be possible to allocate and deallocate resources in a tight loop
> because you either get spurious failures because too many
> unaccounted deallocations are stuck somewhere in the system (and the
> user can't tell that this is due to a race), or you get an OOM
> because the user manages to queue up too much state.
>
> We already have this problem with RLIMIT_NPROC, where waitpid etc.
> return before the process is completely gone. On some
> kernels/configurations, the resulting race is so wide that parallel
> make no longer works reliable because it runs into fork failures.
Or alternatively, use rcu_barrier() occasionally to wait for all
preceding deferred deallocations. And there are quite a few other
ways to take on this problem.
Thanx, Paul
next prev parent reply other threads:[~2017-11-27 14:41 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-10-26 2:26 [PATCH] VFS: use synchronize_rcu_expedited() in namespace_unlock() NeilBrown
2017-10-26 12:27 ` Paul E. McKenney
2017-10-26 13:50 ` Paul E. McKenney
2017-10-27 0:45 ` NeilBrown
2017-10-27 1:24 ` Paul E. McKenney
2017-11-27 11:27 ` Florian Weimer
2017-11-27 14:41 ` Paul E. McKenney [this message]
2017-11-28 22:17 ` NeilBrown
2018-10-05 1:27 ` [PATCH - resend] " NeilBrown
2018-10-05 1:40 ` Al Viro
2018-10-05 2:53 ` NeilBrown
2018-10-05 4:08 ` Paul E. McKenney
2018-11-29 23:33 ` [PATCH - resend*2] " NeilBrown
2018-11-29 23:52 ` Al Viro
2018-11-30 1:09 ` NeilBrown
2018-11-06 3:15 ` [PATCH - resend] " NeilBrown
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