From: Davidlohr Bueso <dave@stgolabs.net>
To: Prateek Sood <prsood@codeaurora.org>
Cc: mingo@kernel.org, longman@redhat.com, peterz@infradead.org,
parri.andrea@gmail.com, linux-kernel@vger.kernel.org,
sramana@codeaurora.org
Subject: Re: [PATCH] rwsem: fix missed wakeup due to reordering of load
Date: Wed, 20 Sep 2017 07:52:54 -0700 [thread overview]
Message-ID: <20170920145254.GA3406@linux-80c1.suse> (raw)
In-Reply-To: <1504794658-15397-1-git-send-email-prsood@codeaurora.org>
On Thu, 07 Sep 2017, Prateek Sood wrote:
> /*
>+ * __rwsem_down_write_failed_common(sem)
>+ * rwsem_optimistic_spin(sem)
>+ * osq_unlock(sem->osq)
>+ * ...
>+ * atomic_long_add_return(&sem->count)
>+ *
>+ * - VS -
>+ *
>+ * __up_write()
>+ * if (atomic_long_sub_return_release(&sem->count) < 0)
>+ * rwsem_wake(sem)
>+ * osq_is_locked(&sem->osq)
>+ *
>+ * And __up_write() must observe !osq_is_locked() when it observes the
>+ * atomic_long_add_return() in order to not miss a wakeup.
>+ *
>+ * This boils down to:
>+ *
>+ * [S.rel] X = 1 [RmW] r0 = (Y += 0)
>+ * MB RMB
>+ * [RmW] Y += 1 [L] r1 = X
>+ *
>+ * exists (r0=1 /\ r1=0)
>+ */
>+ smp_rmb();
Instead, how about just removing the release from atomic_long_sub_return_release()
such that the osq load is not hoisted over the atomic compound (along with Peter's
comment):
diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
index 6c6a2141f271..487ce31078ff 100644
--- a/include/asm-generic/rwsem.h
+++ b/include/asm-generic/rwsem.h
@@ -101,7 +101,7 @@ static inline void __up_read(struct rw_semaphore *sem)
*/
static inline void __up_write(struct rw_semaphore *sem)
{
- if (unlikely(atomic_long_sub_return_release(RWSEM_ACTIVE_WRITE_BIAS,
+ if (unlikely(atomic_long_sub_return(RWSEM_ACTIVE_WRITE_BIAS,
&sem->count) < 0))
rwsem_wake(sem);
}
next prev parent reply other threads:[~2017-09-20 14:53 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-09-07 14:30 [PATCH] rwsem: fix missed wakeup due to reordering of load Prateek Sood
2017-09-19 14:05 ` Andrea Parri
2017-09-20 14:52 ` Davidlohr Bueso [this message]
2017-09-20 21:17 ` Andrea Parri
2017-09-27 21:20 ` Davidlohr Bueso
2017-09-26 18:37 ` Prateek Sood
2017-09-29 9:30 ` [tip:locking/urgent] locking/rwsem-xadd: Fix " tip-bot for Prateek Sood
-- strict thread matches above, loose matches on Subject: below --
2017-08-23 11:28 [PATCH] rwsem: fix " Prateek Sood
2017-08-24 11:29 ` Peter Zijlstra
2017-08-24 12:33 ` Peter Zijlstra
2017-08-24 12:52 ` Peter Zijlstra
2017-09-07 14:08 ` Prateek Sood
2017-07-26 20:17 Prateek Sood
2017-07-27 15:48 ` Waiman Long
2017-07-27 16:59 ` Peter Zijlstra
2017-08-10 8:32 ` Andrea Parri
2017-08-10 10:41 ` Peter Zijlstra
2017-08-10 10:44 ` Peter Zijlstra
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20170920145254.GA3406@linux-80c1.suse \
--to=dave@stgolabs.net \
--cc=linux-kernel@vger.kernel.org \
--cc=longman@redhat.com \
--cc=mingo@kernel.org \
--cc=parri.andrea@gmail.com \
--cc=peterz@infradead.org \
--cc=prsood@codeaurora.org \
--cc=sramana@codeaurora.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).