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[RFC,4/8] seqlock: use atomic writes for the sequence

Message ID 20160919155139.28371-5-alex.bennee@linaro.org
State New
Headers show

Commit Message

Alex Bennée Sept. 19, 2016, 3:51 p.m. UTC 
From: Paolo Bonzini <pbonzini@redhat.com>

There is a data race if the sequence is written concurrently to the
read.  In C11 this has undefined behavior.  Use atomic_set; the
read side is already using atomic_read.

Reported-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
---
 include/qemu/seqlock.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
diff mbox

Patch

diff --git a/include/qemu/seqlock.h b/include/qemu/seqlock.h
index 2e2be4c..8dee11d 100644
--- a/include/qemu/seqlock.h
+++ b/include/qemu/seqlock.h
@@ -31,7 +31,7 @@  static inline void seqlock_init(QemuSeqLock *sl)
 /* Lock out other writers and update the count.  */
 static inline void seqlock_write_begin(QemuSeqLock *sl)
 {
-    ++sl->sequence;
+    atomic_set(&sl->sequence, sl->sequence + 1);
 
     /* Write sequence before updating other fields.  */
     smp_wmb();
@@ -42,7 +42,7 @@  static inline void seqlock_write_end(QemuSeqLock *sl)
     /* Write other fields before finalizing sequence.  */
     smp_wmb();
 
-    ++sl->sequence;
+    atomic_set(&sl->sequence, sl->sequence + 1);
 }
 
 static inline unsigned seqlock_read_begin(QemuSeqLock *sl)