linux-kernel - [PATCH 3.13 103/105] futex: Ensure get_futex_key

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Message-Id: <1414436240-13879-104-git-send-email-kamal@canonical.com>
Date:	Mon, 27 Oct 2014 11:57:18 -0700
From:	Kamal Mostafa <kamal@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Kamal Mostafa <kamal@...onical.com>
Subject: [PATCH 3.13 103/105] futex: Ensure get_futex_key_refs() always implies a barrier

3.13.11.10 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Catalin Marinas <catalin.marinas@....com>

commit 76835b0ebf8a7fe85beb03c75121419a7dec52f0 upstream.

Commit b0c29f79ecea (futexes: Avoid taking the hb->lock if there's
nothing to wake up) changes the futex code to avoid taking a lock when
there are no waiters. This code has been subsequently fixed in commit
11d4616bd07f (futex: revert back to the explicit waiter counting code).
Both the original commit and the fix-up rely on get_futex_key_refs() to
always imply a barrier.

However, for private futexes, none of the cases in the switch statement
of get_futex_key_refs() would be hit and the function completes without
a memory barrier as required before checking the "waiters" in
futex_wake() -> hb_waiters_pending(). The consequence is a race with a
thread waiting on a futex on another CPU, allowing the waker thread to
read "waiters == 0" while the waiter thread to have read "futex_val ==
locked" (in kernel).

Without this fix, the problem (user space deadlocks) can be seen with
Android bionic's mutex implementation on an arm64 multi-cluster system.

Signed-off-by: Catalin Marinas <catalin.marinas@....com>
Reported-by: Matteo Franchin <Matteo.Franchin@....com>
Fixes: b0c29f79ecea (futexes: Avoid taking the hb->lock if there's nothing to wake up)
Acked-by: Davidlohr Bueso <dave@...olabs.net>
Tested-by: Mike Galbraith <umgwanakikbuti@...il.com>
Cc: Darren Hart <dvhart@...ux.intel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Kamal Mostafa <kamal@...onical.com>
---
 kernel/futex.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/futex.c b/kernel/futex.c
index 4741b1f..2b1583e 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -194,6 +194,8 @@ static void get_futex_key_refs(union futex_key *key)
 	case FUT_OFF_MMSHARED:
 		atomic_inc(&key->private.mm->mm_count);
 		break;
+	default:
+		smp_mb(); /* explicit MB (B) */
 	}
 }

-- 
1.9.1

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