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author | Catalin Marinas <catalin.marinas@arm.com> | 2014-10-17 20:38:49 +0400 |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2014-10-18 20:28:51 +0400 |
commit | 76835b0ebf8a7fe85beb03c75121419a7dec52f0 (patch) | |
tree | ba3c00f197cdf77c9a43ec03828ed474931fc040 /fs | |
parent | 0429fbc0bdc297d64188483ba029a23773ae07b0 (diff) | |
download | linux-76835b0ebf8a7fe85beb03c75121419a7dec52f0.tar.xz |
futex: Ensure get_futex_key_refs() always implies a barrier
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@arm.com>
Reported-by: Matteo Franchin <Matteo.Franchin@arm.com>
Fixes: b0c29f79ecea (futexes: Avoid taking the hb->lock if there's nothing to wake up)
Acked-by: Davidlohr Bueso <dave@stgolabs.net>
Tested-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: <stable@vger.kernel.org>
Cc: Darren Hart <dvhart@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'fs')
0 files changed, 0 insertions, 0 deletions