asm-generic/futex: Re-enable preemption in futex_atomic_cmpxchg_inatomic()

The recent decoupling of pagefault disable and preempt disable added an
explicit preempt_disable/enable() pair to the futex_atomic_cmpxchg_inatomic()
implementation in asm-generic/futex.h. But it forgot to add preempt_enable()
calls to the error handling code pathes, which results in a preemption count
imbalance.

This is observable on boot when the test for atomic_cmpxchg() is calling
futex_atomic_cmpxchg_inatomic() on a NULL pointer.

Add the missing preempt_enable() calls to the error handling code pathes.

[ tglx: Massaged changelog ]

Fixes: d9b9ff8c1889 ("sched/preempt, futex: Disable preemption in UP futex_atomic_cmpxchg_inatomic() explicitly")
Signed-off-by: Romain Perier <romain.perier@free-electrons.com>
Cc: linux-arch@vger.kernel.org
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/1460640963-690-1-git-send-email-romain.perier@free-electrons.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 6 insertions, 2 deletions

diff --git a/include/asm-generic/futex.h b/include/asm-generic/futex.h
index e56272c919b5..bf2d34c9d804 100644
--- a/include/asm-generic/futex.h
+++ b/include/asm-generic/futex.h
@@ -108,11 +108,15 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
         u32 val;
 
         preempt_disable();
-        if (unlikely(get_user(val, uaddr) != 0))
+        if (unlikely(get_user(val, uaddr) != 0)) {
+                preempt_enable();
                 return -EFAULT;
+        }
 
-        if (val == oldval && unlikely(put_user(newval, uaddr) != 0))
+        if (val == oldval && unlikely(put_user(newval, uaddr) != 0)) {
+                preempt_enable();
                 return -EFAULT;
+        }
 
         *uval = val;
         preempt_enable();