linux-kernel - [PATCH 6/6] locking/rwsem: Use the force

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Date:   Thu, 23 Feb 2023 13:26:48 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     longman@...hat.com, mingo@...hat.com, will@...nel.org
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        boqun.feng@...il.com
Subject: [PATCH 6/6] locking/rwsem: Use the force

Now that the writer adjustment is done from the wakeup side and
HANDOFF guarantees spinning/stealing is disabled, use the combined
guarantee it ignore spurious READER_BIAS and directly claim the lock.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/locking/lock_events_list.h |    1 +
 kernel/locking/rwsem.c            |   21 +++++++++++++++++++++
 2 files changed, 22 insertions(+)

--- a/kernel/locking/lock_events_list.h
+++ b/kernel/locking/lock_events_list.h
@@ -67,3 +67,4 @@ LOCK_EVENT(rwsem_rlock_handoff)	/* # of
 LOCK_EVENT(rwsem_wlock)		/* # of write locks acquired		*/
 LOCK_EVENT(rwsem_wlock_fail)	/* # of failed write lock acquisitions	*/
 LOCK_EVENT(rwsem_wlock_handoff)	/* # of write lock handoffs		*/
+LOCK_EVENT(rwsem_wlock_ehandoff) /* # of write lock early handoffs	*/
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -433,6 +433,26 @@ static void rwsem_writer_wake(struct rw_
 	lockdep_assert_held(&sem->wait_lock);
 
 	count = atomic_long_read(&sem->count);
+
+	/*
+	 * Since rwsem_mark_wake() is only called (with WAKE_ANY) when
+	 * the lock is unlocked, and the HANDOFF bit guarantees that
+	 * all spinning / stealing is disabled, it is posssible to
+	 * unconditionally claim the lock -- any READER_BIAS will be
+	 * temporary.
+	 */
+	if (count & RWSEM_FLAG_HANDOFF) {
+		unsigned long adjustment = RWSEM_WRITER_LOCKED - RWSEM_FLAG_HANDOFF;
+
+		if (list_is_singular(&sem->wait_list))
+			adjustment -= RWSEM_FLAG_WAITERS;
+
+		atomic_long_set(&sem->owner, (long)waiter->task);
+		atomic_long_add(adjustment, &sem->count);
+		lockevent_inc(rwsem_wlock_ehandoff);
+		goto success;
+	}
+
 	do {
 		bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
 
@@ -479,6 +499,7 @@ static void rwsem_writer_wake(struct rw_
 		return;
 	}
 
+success:
 	/*
 	 * Have rwsem_writer_wake() fully imply rwsem_del_waiter() on
 	 * success.