Re: [PATCH v3 1/2] locking/qrwlock: Better optimization for interrupt context readers

From: Waiman Long
Date: Wed Jun 17 2015 - 21:30:26 EST

On 06/16/2015 08:17 AM, Will Deacon wrote:
Hi Waiman,

On Mon, Jun 15, 2015 at 11:24:02PM +0100, Waiman Long wrote:
The qrwlock is fair in the process context, but becoming unfair when
in the interrupt context to support use cases like the tasklist_lock.

The current code isn't that well-documented on what happens when
in the interrupt context. The rspin_until_writer_unlock() will only
spin if the writer has gotten the lock. If the writer is still in the
waiting state, the increment in the reader count will cause the writer
to remain in the waiting state and the new interrupt context reader
will get the lock and return immediately. The current code, however,
do an additional read of the lock value which is not necessary as the
information have already been there in the fast path. This may sometime
cause an additional cacheline load when the lock is highly contended.

This patch passes the lock value information gotten in the fast path
to the slow path to eliminate the additional read. It also clarify the
action for the interrupt context readers more explicitly.

Signed-off-by: Waiman Long<Waiman.Long@xxxxxx>
include/asm-generic/qrwlock.h | 4 ++--
kernel/locking/qrwlock.c | 14 ++++++++------
2 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index 00c12bb..d7d7557 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -43,22 +43,24 @@ rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts)
* queue_read_lock_slowpath - acquire read lock of a queue rwlock
* @lock: Pointer to queue rwlock structure
-void queue_read_lock_slowpath(struct qrwlock *lock)
+void queue_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
- u32 cnts;
* Readers come here when they cannot get the lock without waiting
if (unlikely(in_interrupt())) {
- * Readers in interrupt context will spin until the lock is
- * available without waiting in the queue.
+ * Readers in interrupt context will get the lock immediately
+ * if the writer is just waiting (not holding the lock yet)
+ * or they will spin until the lock is available without
+ * waiting in the queue.
- cnts = smp_load_acquire((u32 *)&lock->cnts);
+ if ((cnts& _QW_WMASK) != _QW_LOCKED)
+ return;
I really doubt the check here is gaining you any performance, given
rspin_until_write_unlock does the same check immediately and should be
inlined. Just dropping the acquire and passing cnts through should be

Yes, you are right. I can just pass the cnt to rspin_until_write_unlock() and be done with it.

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