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Subject[PATCH v6 06/46] percpu_rwlock: Rearrange the read-lock code to fastpath nested percpu readers
Date
If we are dealing with a nester percpu reader, we can optimize away quite
a few costly operations. Improve that fastpath further, by avoiding the
unnecessary addition and subtraction of 'READER_PRESENT' to reader_refcnt,
by rearranging the code a bit.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

lib/percpu-rwlock.c | 14 ++++++--------
1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/lib/percpu-rwlock.c b/lib/percpu-rwlock.c
index ce7e440..ed36531 100644
--- a/lib/percpu-rwlock.c
+++ b/lib/percpu-rwlock.c
@@ -80,23 +80,21 @@ void percpu_read_lock_irqsafe(struct percpu_rwlock *pcpu_rwlock)
preempt_disable();

/*
- * Let the writer know that a reader is active, even before we choose
- * our reader-side synchronization scheme.
- */
- this_cpu_add(pcpu_rwlock->rw_state->reader_refcnt, READER_PRESENT);
-
- /*
* If we are already using per-cpu refcounts, it is not safe to switch
* the synchronization scheme. So continue using the refcounts.
*/
if (reader_uses_percpu_refcnt(pcpu_rwlock)) {
this_cpu_inc(pcpu_rwlock->rw_state->reader_refcnt);
- this_cpu_sub(pcpu_rwlock->rw_state->reader_refcnt,
- READER_PRESENT);
return;
}

/*
+ * Let the writer know that a reader is active, even before we choose
+ * our reader-side synchronization scheme.
+ */
+ this_cpu_add(pcpu_rwlock->rw_state->reader_refcnt, READER_PRESENT);
+
+ /*
* The write to 'reader_refcnt' must be visible before we read
* 'writer_signal'.
*/


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