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Re: [PATCH] seqlock: fix seqlock_write_unlock_impl function

From: Philippe Mathieu-Daudé
Subject: Re: [PATCH] seqlock: fix seqlock_write_unlock_impl function
Date: Wed, 29 Jan 2020 16:49:27 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2

On 1/29/20 3:49 PM, Luc Michel wrote:
The seqlock write unlock function was incorrectly calling
seqlock_write_begin() instead of seqlock_write_end(), and was releasing
the lock before incrementing the sequence. This could lead to a race
condition and a corrupted sequence number becoming odd even though the
lock is not held.

I'm surprised it took 18 months to figure this out.

Fixes: 988fcafc730
Reviewed-by: Philippe Mathieu-Daudé <address@hidden>

Signed-off-by: Luc Michel <address@hidden>
  include/qemu/seqlock.h | 4 ++--
  1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/qemu/seqlock.h b/include/qemu/seqlock.h
index fd408b7ec5..8b6b4ee4bb 100644
--- a/include/qemu/seqlock.h
+++ b/include/qemu/seqlock.h
@@ -53,15 +53,15 @@ static inline void seqlock_write_lock_impl(QemuSeqLock *sl, 
QemuLockable *lock)
  #define seqlock_write_lock(sl, lock) \
      seqlock_write_lock_impl(sl, QEMU_MAKE_LOCKABLE(lock))
-/* Lock out other writers and update the count. */
+/* Update the count and release the lock.  */
  static inline void seqlock_write_unlock_impl(QemuSeqLock *sl, QemuLockable 
+    seqlock_write_end(sl);
-    seqlock_write_begin(sl);
  #define seqlock_write_unlock(sl, lock) \
      seqlock_write_unlock_impl(sl, QEMU_MAKE_LOCKABLE(lock))

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