From: Rusty Russell <rusty@rustcorp.com.au>
To: Linus Torvalds <torvalds@osdl.org>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: Jamie Lokier <jamie@shareable.org>, Andrew Morton <akpm@osdl.org>,
Ingo Molnar <mingo@redhat.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Alternate futex non-page-pinning and COW fix
Date: Thu, 04 Sep 2003 12:43:04 +1000 [thread overview]
Message-ID: <20030904024345.CC3E02C018@lists.samba.org> (raw)
In-Reply-To: Your message of "Wed, 03 Sep 2003 12:05:36 MST." <Pine.LNX.4.44.0309031201170.31853-100000@home.osdl.org>
In message <Pine.LNX.4.44.0309031201170.31853-100000@home.osdl.org> you write:
> Private mappings that haven't been broken by COW (and a read-only mapping
> never will be) will see updates as they happen on the file that backs it.
> That's the fundamental difference between "mmap(MAP_PRIVATE)" and
> "read()".
Right, so it would be consistent for someone doing a FUTEX_WAIT on an
"intact" (not broken by COW) private mapping to see a FUTEX_WAKE done
on that file.
However, Jamie's futex code will see !VM_SHARED on the mapping, and
compare futexes by mm + uaddr (rather than inode + file offset), so
this is NOT the case. Using VM_MAYSHARE instead would make the
MAP_SHARED readonly case work as above, though.
The way futexes are used now, they're both "don't care". If you have
a private mapping or read-only mapping, you'll never get woken by
others with the same file mapped writable shared, but WTF were you
waiting for a futex if the mapping is private anyway: the lock
acquisition won't work (and sleeping forever is easier to debug than
two tasks getting the lock).
Oh no, I think I'm starting to understand the VM a little.
Ick.
Rusty.
--
Anyone who quotes me in their sig is an idiot. -- Rusty Russell.
next prev parent reply other threads:[~2003-09-04 2:44 UTC|newest]
Thread overview: 102+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-08-26 3:12 [PATCH 2/2] Futex non-page-pinning fix Rusty Russell
2003-08-26 4:06 ` Andrew Morton
2003-08-26 5:30 ` Ingo Molnar
2003-08-26 5:50 ` Andrew Morton
2003-08-26 5:58 ` Ingo Molnar
2003-08-26 6:14 ` Andrew Morton
2003-08-26 6:36 ` Ingo Molnar
2003-08-26 7:02 ` Andrew Morton
2003-08-26 7:56 ` Arjan van de Ven
2003-08-26 8:08 ` Muli Ben-Yehuda
2003-08-26 8:11 ` Arjan van de Ven
2003-08-26 8:25 ` Andrew Morton
2003-08-26 9:02 ` Muli Ben-Yehuda
2003-08-26 10:38 ` William Lee Irwin III
2003-08-26 10:44 ` Andrew Morton
2003-08-26 10:45 ` William Lee Irwin III
2003-08-26 17:29 ` Andrew Morton
2003-08-26 19:35 ` William Lee Irwin III
2003-08-27 5:17 ` Rusty Russell
2003-08-27 6:20 ` Andrew Morton
2003-08-28 0:47 ` Rusty Russell
2003-08-28 8:21 ` Andrew Morton
2003-08-29 3:46 ` Rusty Russell
2003-08-29 4:17 ` Andrew Morton
2003-08-30 7:49 ` Rusty Russell
2003-09-01 0:35 ` Jamie Lokier
2003-09-01 4:11 ` Rusty Russell
2003-09-01 20:57 ` Hugh Dickins
2003-09-02 3:12 ` Rusty Russell
2003-09-02 6:51 ` Jamie Lokier
2003-09-02 16:14 ` Hugh Dickins
2003-09-02 19:54 ` Jamie Lokier
2003-09-02 20:15 ` Andrew Morton
2003-09-02 21:20 ` Jamie Lokier
2003-09-03 2:40 ` Rusty Russell
2003-09-03 7:36 ` [PATCH] Alternate futex non-page-pinning and COW fix Jamie Lokier
2003-09-03 11:19 ` Hugh Dickins
2003-09-03 14:38 ` Jamie Lokier
2003-09-03 17:39 ` Jamie Lokier
2003-09-03 17:55 ` Linus Torvalds
2003-09-03 18:06 ` Hugh Dickins
2003-09-03 18:19 ` Linus Torvalds
2003-09-03 18:43 ` Hugh Dickins
2003-09-03 19:05 ` Linus Torvalds
2003-09-03 19:40 ` Hugh Dickins
2003-09-03 20:04 ` Linus Torvalds
2003-09-04 2:43 ` Rusty Russell [this message]
2003-09-04 8:28 ` Linus Torvalds
2003-09-04 12:20 ` Hugh Dickins
2003-09-04 15:40 ` Linus Torvalds
2003-09-04 16:55 ` Hugh Dickins
2003-09-04 18:38 ` Jamie Lokier
2003-09-04 18:46 ` Linus Torvalds
2003-09-04 20:04 ` Jamie Lokier
2003-09-04 21:49 ` Linus Torvalds
2003-09-04 22:10 ` Jamie Lokier
2003-09-04 17:16 ` Jamie Lokier
2003-09-04 17:38 ` Linus Torvalds
2003-09-04 18:42 ` Linus Torvalds
2003-09-04 18:42 ` Linus Torvalds
2003-09-05 3:55 ` Rusty Russell
2003-09-05 17:55 ` Jamie Lokier
2003-09-04 17:26 ` Jamie Lokier
2003-09-04 1:30 ` Rusty Russell
2003-09-04 21:00 ` Jamie Lokier
2003-09-05 5:19 ` Rusty Russell
2003-09-05 20:54 ` Jamie Lokier
2003-09-07 6:45 ` Rusty Russell
2003-09-07 13:20 ` Jamie Lokier
2003-09-08 1:49 ` Rusty Russell
2003-09-08 9:44 ` Jamie Lokier
2003-09-03 14:40 ` [PATCH 2] Little fixes to previous futex patch Jamie Lokier
2003-09-04 16:45 ` Hugh Dickins
2003-09-04 17:59 ` Jamie Lokier
2003-09-04 18:35 ` Hugh Dickins
2003-09-04 20:11 ` Jamie Lokier
2003-09-04 21:36 ` Hugh Dickins
2003-09-04 21:58 ` Jamie Lokier
2003-09-07 7:23 ` Ingo Molnar
2003-09-07 12:27 ` Hugh Dickins
2003-09-07 15:03 ` Jamie Lokier
2003-09-08 1:56 ` Rusty Russell
2003-09-07 13:00 ` Jamie Lokier
2003-09-08 3:32 ` Ingo Molnar
2003-09-08 9:33 ` Jamie Lokier
2003-09-08 9:57 ` Ingo Molnar
2003-09-05 4:56 ` Rusty Russell
2003-09-03 15:34 ` [PATCH] Alternate futex non-page-pinning and COW fix Andrew Morton
2003-09-03 17:16 ` Jamie Lokier
2003-09-04 1:35 ` Rusty Russell
2003-09-04 17:35 ` Jamie Lokier
2003-09-03 0:14 ` [PATCH 2/2] Futex non-page-pinning fix Rusty Russell
2003-09-03 1:16 ` Andrew Morton
2003-09-03 1:54 ` Dave Hansen
2003-09-03 2:54 ` Andrew Morton
2003-09-02 3:23 ` Hugh Dickins
2003-09-02 23:58 ` Rusty Russell
2003-08-27 8:37 ` Hugh Dickins
2003-08-27 8:56 ` William Lee Irwin III
2003-08-27 10:38 ` Andrew Morton
2003-08-27 10:57 ` Hugh Dickins
2003-08-28 8:03 ` Rusty Russell
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