Re: [PATCH] locking/rwsem: Allow slowpath writer to ignore handoff bit if not set by first waiter

From: Waiman Long
Date: Wed Jun 22 2022 - 16:07:53 EST


On 6/22/22 13:48, john.p.donnelly@xxxxxxxxxx wrote:
On 4/27/22 8:23 PM, Hillf Danton wrote:
On  Wed, 27 Apr 2022 13:31:24 -0400 Waiman Long wrote:
With commit d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling more
consistent"), the writer that sets the handoff bit can be interrupted
out without clearing the bit if the wait queue isn't empty. This disables
reader and writer optimistic lock spinning and stealing.

Now if a non-first writer in the queue is somehow woken up or first
entering the waiting loop, it can't acquire the lock.  This is not
the case before that commit as the writer that set the handoff bit
will clear it when exiting out via the out_nolock path. This is less
efficient as the busy rwsem stays in an unlock state for a longer time.

This patch allows a non-first writer to ignore the handoff bit if it
is not originally set or initiated by the first waiter.

Fixes: d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling more consistent")
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
  kernel/locking/rwsem.c | 30 ++++++++++++++++++++----------
  1 file changed, 20 insertions(+), 10 deletions(-)

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 9d1db4a54d34..65f0262f635e 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -335,8 +335,6 @@ struct rwsem_waiter {
      struct task_struct *task;
      enum rwsem_waiter_type type;
      unsigned long timeout;
-
-    /* Writer only, not initialized in reader */
      bool handoff_set;
  };
  #define rwsem_first_waiter(sem) \
@@ -459,10 +457,12 @@ static void rwsem_mark_wake(struct rw_semaphore *sem,
               * to give up the lock), request a HANDOFF to
               * force the issue.
               */
-            if (!(oldcount & RWSEM_FLAG_HANDOFF) &&
-                time_after(jiffies, waiter->timeout)) {
-                adjustment -= RWSEM_FLAG_HANDOFF;
-                lockevent_inc(rwsem_rlock_handoff);
+            if (time_after(jiffies, waiter->timeout)) {
+                if (!(oldcount & RWSEM_FLAG_HANDOFF)) {
+                    adjustment -= RWSEM_FLAG_HANDOFF;
+                    lockevent_inc(rwsem_rlock_handoff);
+                }
+                waiter->handoff_set = true;
              }

Handoff is tracked in both sem->count and waiter->handoff_set,

atomic_long_add(-adjustment, &sem->count);
@@ -599,7 +599,7 @@ rwsem_del_wake_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter,
  static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
                      struct rwsem_waiter *waiter)
  {
-    bool first = rwsem_first_waiter(sem) == waiter;
+    struct rwsem_waiter *first = rwsem_first_waiter(sem);
      long count, new;
        lockdep_assert_held(&sem->wait_lock);
@@ -609,11 +609,20 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
          bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
            if (has_handoff) {
-            if (!first)
+            /*
+             * Honor handoff bit and yield only when the first
+             * waiter is the one that set it. Otherwisee, we
+             * still try to acquire the rwsem.
+             */
+            if (first->handoff_set && (waiter != first))
                  return false;

and checked against both parties, thus in a simpler manner RWSEM_FLAG_HANDOFF
in sem->count means the first waiter has been waiting for lock long enough.

Feel free to ignore the comment given the Fixes tag above.

Hillf
  -            /* First waiter inherits a previously set handoff bit */
-            waiter->handoff_set = true;
+            /*
+             * First waiter can inherit a previously set handoff
+             * bit and spin on rwsem if lock acquisition fails.
+             */
+            if (waiter == first)
+                waiter->handoff_set = true;
          }
            new = count;
@@ -1027,6 +1036,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat
      waiter.task = current;
      waiter.type = RWSEM_WAITING_FOR_READ;
      waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
+    waiter.handoff_set = false;
        raw_spin_lock_irq(&sem->wait_lock);
      if (list_empty(&sem->wait_list)) {
--
2.27.0


Was this ever added ?

I don't see it in


a111daf0c53ae 2022-06-19 | Linux 5.19-rc3

This patch hasn't been taken up by upstream yet. I have reposted a v2 with update to the patch description.

Cheers,
Longman