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seqlock: read sequence number atomically 

With this change we make sure that the compiler will not
optimise the read of the sequence number in any way.

Signed-off-by: Emilio G. Cota <cota@braap.org>
Message-Id: <1440375847-17603-8-git-send-email-cota@braap.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Emilio G. Cota 2015-08-23 20:23:36 -04:00 committed by Paolo Bonzini
parent 123fdbac9b
commit d12f730948
1 changed files with 3 additions and 3 deletions
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6
include/qemu/seqlock.h
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@ -55,18 +55,18 @@ static inline void seqlock_write_unlock(QemuSeqLock *sl)
static inline unsigned seqlock_read_begin(QemuSeqLock *sl) static inline unsigned seqlock_read_begin(QemuSeqLock *sl)
{ {
/* Always fail if a write is in progress. */ /* Always fail if a write is in progress. */
unsigned ret = sl->sequence & ~1; unsigned ret = atomic_read(&sl->sequence);
/* Read sequence before reading other fields. */ /* Read sequence before reading other fields. */
smp_rmb(); smp_rmb();
return ret; return ret & ~1;
} }
static inline int seqlock_read_retry(const QemuSeqLock *sl, unsigned start) static inline int seqlock_read_retry(const QemuSeqLock *sl, unsigned start)
{ {
/* Read other fields before reading final sequence. */ /* Read other fields before reading final sequence. */
smp_rmb(); smp_rmb();
return unlikely(sl->sequence != start); return unlikely(atomic_read(&sl->sequence) != start);
} }
#endif #endif