linux-kernel - [PATCH -v2 4/7] locking/percpu-rwsem: Extract __percpu_down_read

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Message-Id: <20200131151540.098485539@infradead.org>
Date:   Fri, 31 Jan 2020 16:07:07 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     peterz@...radead.org, mingo@...nel.org, will@...nel.org
Cc:     oleg@...hat.com, tglx@...utronix.de, linux-kernel@...r.kernel.org,
        bigeasy@...utronix.de, juri.lelli@...hat.com, williams@...hat.com,
        bristot@...hat.com, longman@...hat.com, dave@...olabs.net,
        jack@...e.com
Subject: [PATCH -v2 4/7] 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@...radead.org>
Tested-by: Juri Lelli <juri.lelli@...hat.com>
---
 kernel/locking/percpu-rwsem.c |   19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -45,7 +45,7 @@ void percpu_free_rwsem(struct percpu_rw_
 }
 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
 	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;