[tip: locking/core] locking/percpu-rwsem: Extract __percpu_down_read_trylock()

["tip-bot2 for Peter Zijlstra" <tip-bot2@xxxxxxxxxxxxx>]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     75ff64572e497578e238fefbdff221c96f29067a
Gitweb:        https://git.kernel.org/tip/75ff64572e497578e238fefbdff221c96f29067a
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Thu, 31 Oct 2019 12:34:23 +01:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Tue, 11 Feb 2020 13:10:55 +01:00

locking/percpu-rwsem: Extract __percpu_down_read_trylock()

In preparation for removing the embedded rwsem and building a custom
lock, extract the read-trylock primitive.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Reviewed-by: Davidlohr Bueso <dbueso@xxxxxxx>
Acked-by: Will Deacon <will@xxxxxxxxxx>
Acked-by: Waiman Long <longman@xxxxxxxxxx>
Tested-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200131151540.098485539@xxxxxxxxxxxxx
---
 kernel/locking/percpu-rwsem.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index becf925..b155e8e 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -45,7 +45,7 @@ void percpu_free_rwsem(struct percpu_rw_semaphore *sem)
 }
 EXPORT_SYMBOL_GPL(percpu_free_rwsem);
 
-bool __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
+static bool __percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
 {
 	__this_cpu_inc(*sem->read_count);
 
@@ -73,11 +73,18 @@ bool __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
 	if (likely(!smp_load_acquire(&sem->readers_block)))
 		return true;
 
-	/*
-	 * Per the above comment; we still have preemption disabled and
-	 * will thus decrement on the same CPU as we incremented.
-	 */
-	__percpu_up_read(sem);
+	__this_cpu_dec(*sem->read_count);
+
+	/* Prod writer to re-evaluate readers_active_check() */
+	rcuwait_wake_up(&sem->writer);
+
+	return false;
+}
+
+bool __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
+{
+	if (__percpu_down_read_trylock(sem))
+		return true;
 
 	if (try)
 		return false;