From: Siddhesh Poyarekar <siddhesh.poyarekar@gmail.com>
To: linux-kernel@vger.kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>,
Darren Hart <dvhart@linux.intel.com>,
Siddhesh Poyarekar <siddhesh.poyarekar@gmail.com>
Subject: [PATCH] [RESEND 2] Take over futex of dead task only if FUTEX_WAITERS is not set
Date: Mon, 22 Oct 2012 08:50:34 +0530 [thread overview]
Message-ID: <1350876034-22023-1-git-send-email-siddhesh.poyarekar@gmail.com> (raw)
In-Reply-To: <CAAHN_R1O3asyJGXqjs5qt0b_qGR6S6RuM+E_85VaEoWAyAMheQ@mail.gmail.com>
In futex_lock_pi_atomic, we consider that if the value in the futex
variable is 0 with additional flags, then it is safe for takeover
since the owner of the futex is dead. However, when FUTEX_WAITERS is
set in the futex value, handle_futex_death calls futex_wake to wake up
one task. Hence the assumption in futex_lock_pi_atomic is not correct.
The correct assumption is that a futex may be considered safe for a
takeover if The FUTEX_OWNER_DIED bit is set, the TID bits are 0 and
the FUTEX_WAITERS bit is not set.
The race described above can be seen in the reproducer in the
following glibc bug report:
http://sourceware.org/bugzilla/show_bug.cgi?id=14076
Signed-off-by: Siddhesh Poyarekar <siddhesh.poyarekar@gmail.com>
---
kernel/futex.c | 7 ++++++-
1 files changed, 6 insertions(+), 1 deletions(-)
diff --git a/kernel/futex.c b/kernel/futex.c
index 3717e7b..9aa2d5a 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -760,9 +760,14 @@ retry:
* case. We also do an unconditional take over, when the owner
* of the futex died.
*
+ * We do not take over the futex if FUTEX_WAITERS is set because we
+ * could end up waking two tasks, the current one and the one that the
+ * futex death event wakes in handle_futex_death.
+ *
* This is safe as we are protected by the hash bucket lock !
*/
- if (unlikely(ownerdied || !(curval & FUTEX_TID_MASK))) {
+ if (unlikely(ownerdied ||
+ !(curval & (FUTEX_TID_MASK | FUTEX_WAITERS)))) {
/* Keep the OWNER_DIED bit */
newval = (curval & ~FUTEX_TID_MASK) | vpid;
ownerdied = 0;
--
1.7.7.6
next prev parent reply other threads:[~2012-10-22 3:21 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-11 14:52 [PATCH] Take over futex of dead task only if FUTEX_WAITERS is not set Siddhesh Poyarekar
2012-10-17 7:15 ` [PATCH RESEND] " Siddhesh Poyarekar
2012-10-22 3:20 ` Siddhesh Poyarekar [this message]
2012-10-23 14:04 ` [PATCH] [RESEND 2] " Darren Hart
2012-10-23 20:29 ` Thomas Gleixner
2012-10-24 12:48 ` Siddhesh Poyarekar
2012-10-24 18:08 ` Thomas Gleixner
2012-10-25 4:36 ` Darren Hart
2012-10-25 4:44 ` Siddhesh Poyarekar
2012-10-25 4:33 ` Darren Hart
2012-10-25 8:14 ` Thomas Gleixner
2012-10-25 8:18 ` Darren Hart
2012-11-01 21:35 ` [tip:core/urgent] futex: Handle futex_pi OWNER_DIED take over correctly tip-bot for Thomas Gleixner
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