From: Al Viro <viro@zeniv.linux.org.uk>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: "Paul E. McKenney" <paulmck@kernel.org>,
parri.andrea@gmail.com, will@kernel.org, peterz@infradead.org,
boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com,
j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com,
dlustig@nvidia.com, joel@joelfernandes.org,
linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org
Subject: Re: Litmus test for question from Al Viro
Date: Thu, 1 Oct 2020 20:29:58 +0100 [thread overview]
Message-ID: <20201001192958.GH3421308@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20201001183925.GA259470@rowland.harvard.edu>
On Thu, Oct 01, 2020 at 02:39:25PM -0400, Alan Stern wrote:
> The problem with a plain write is that it isn't guaranteed to be atomic
> in any sense. In principle, the compiler could generate code for CPU1
> which would write 0 to V->A more than once.
>
> Although I strongly doubt that any real compiler would actually do this,
> the memory model does allow for it, out of an overabundance of caution.
Point... OK, not a problem - actually there will be WRITE_ONCE() for other
reasons; the real-life (pseudo-)code is
spin_lock(&file->f_lock);
to_free = NULL;
head = file->f_ep;
if (head->first == &epitem->fllink && epitem->fllink.next == NULL) {
/* the set will go empty */
file->f_ep = NULL;
if (!is_file_epoll(file)) {
/*
* not embedded into struct eventpoll; we want it
* freed unless it's on the check list, in which
* case we leave it for reverse path check to free.
*/
v = container_of(head, struct ep_head, epitems);
if (!smp_load_acquire(&v->next))
to_free = v;
}
}
hlist_del_rcu(&epitem->fllink);
spin_unlock(file->f_lock);
kfree(to_free);
and hlist_del_rcu() will use WRITE_ONCE() to store the updated forward links.
That goes into ep_remove() and CPU1 side of that thing is the final (set-emptying)
call. CPU2 side is the list traversal step in reverse_path_check() and
in clear_tfile_check_list():
// under rcu_read_lock()
to_free = head;
epitem = rcu_dereference(hlist_first_rcu(&head->epitems));
if (epitem) {
spin_lock(&epitem->file->f_lock);
if (!hlist_empty(&head->epitems))
to_free = NULL;
head->next = NULL;
spin_unlock(&epitem->file->f_lock);
}
kfree(to_free);
next prev parent reply other threads:[~2020-10-01 19:30 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-01 4:51 Litmus test for question from Al Viro Paul E. McKenney
2020-10-01 16:15 ` Alan Stern
2020-10-01 16:36 ` Al Viro
2020-10-01 18:39 ` Alan Stern
2020-10-01 19:29 ` Al Viro [this message]
2020-10-01 21:30 ` Paul E. McKenney
2020-10-03 2:01 ` Alan Stern
2020-10-03 13:22 ` Alan Stern
2020-10-03 15:16 ` Akira Yokosawa
2020-10-03 17:13 ` Bug in herd7 [Was: Re: Litmus test for question from Al Viro] Alan Stern
2020-10-03 22:50 ` Akira Yokosawa
2020-10-04 1:40 ` [PATCH] tools: memory-model: Document that the LKMM can easily miss control dependencies Alan Stern
2020-10-04 21:07 ` joel
2020-10-04 23:12 ` Paul E. McKenney
2020-10-05 15:15 ` Bug in herd7 [Was: Re: Litmus test for question from Al Viro] Luc Maranget
2020-10-05 15:53 ` Alan Stern
2020-10-05 16:52 ` Paul E. McKenney
2020-10-05 18:19 ` Alan Stern
2020-10-05 19:18 ` Paul E. McKenney
2020-10-05 19:48 ` Alan Stern
2020-10-06 16:39 ` Paul E. McKenney
2020-10-06 17:05 ` Alan Stern
2020-10-07 17:50 ` Paul E. McKenney
2020-10-07 19:40 ` Alan Stern
2020-10-07 22:38 ` Paul E. McKenney
2020-10-08 2:25 ` Alan Stern
2020-10-08 2:50 ` Paul E. McKenney
2020-10-08 14:01 ` Alan Stern
2020-10-08 18:32 ` Paul E. McKenney
2020-10-05 15:54 ` Paul E. McKenney
2020-10-04 23:31 ` Litmus test for question from Al Viro Paul E. McKenney
2020-10-05 2:38 ` Alan Stern
2020-10-05 8:20 ` Will Deacon
2020-10-05 9:12 ` Will Deacon
2020-10-05 14:01 ` Paul E. McKenney
2020-10-05 14:23 ` Alan Stern
2020-10-05 15:13 ` Will Deacon
2020-10-05 15:16 ` Alan Stern
2020-10-05 15:35 ` Peter Zijlstra
2020-10-05 15:49 ` Paul E. McKenney
2020-10-05 14:16 ` Alan Stern
2020-10-05 14:03 ` Paul E. McKenney
2020-10-05 14:24 ` Alan Stern
2020-10-05 14:44 ` joel
2020-10-05 15:55 ` Paul E. McKenney
2020-10-05 8:36 ` David Laight
2020-10-05 13:59 ` Paul E. McKenney
2020-10-03 16:08 ` joel
2020-10-03 16:11 ` joel
2020-10-04 23:13 ` Paul E. McKenney
2020-10-03 2:35 ` Jon Masters
2020-10-04 23:32 ` Paul E. McKenney
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