[PATCH 10/10] seqlock: Add do_read_seqcount_retry() in front of seqcount_lockdep_reader_access() in read_seqbegin()

[jiasheng]

We have found that in the complied files seqcount_lockdep_reader_access()
appear more than 20 times, and under at least 90% circumstances
that do_read_seqcount_retry() and seqcount_lockdep_reader_access()
appear in pairs.
For example, they appear together in the irqfd_wakeup()
of the file complie from 'virt/kvm/eventfd.c'.
But we have found that in the read_seqbegin(), there is only
seqcount_lockdep_reader_access() instead of the pair.
Therefore, we consider that the do_read_seqcount_retry()
might be forgotten.

Signed-off-by: jiasheng <jiasheng@xxxxxxxxxxx>
---
 include/linux/seqlock.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index f61e34f..14169ce 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -837,8 +837,10 @@ typedef struct {
  */
 static inline unsigned read_seqbegin(const seqlock_t *sl)
 {
-	unsigned ret = read_seqcount_begin(&sl->seqcount);
-
+	unsigned int ret = 0;
+
+	if (read_seqcount_retry(&sl->seqcount, ret))
+		ret = read_seqcount_begin(&sl->seqcount);
 	kcsan_atomic_next(0);  /* non-raw usage, assume closing read_seqretry() */
 	kcsan_flat_atomic_begin();
 	return ret;
-- 
2.7.4