linux-kernel - [PATCH 4/6] locking/rwsem: Split out rwsem_reader

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Date:   Thu, 23 Feb 2023 13:26:46 +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 4/6] locking/rwsem: Split out rwsem_reader_wake()

To provide symmetry with rwsem_writer_wake().

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/locking/rwsem.c |   84 +++++++++++++++++++++++++++----------------------
 1 file changed, 47 insertions(+), 37 deletions(-)

--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -106,9 +106,9 @@
  * atomic_long_cmpxchg() will be used to obtain writer lock.
  *
  * There are three places where the lock handoff bit may be set or cleared.
- * 1) rwsem_mark_wake() for readers		-- set, clear
+ * 1) rwsem_reader_wake() for readers	-- set, clear
  * 2) rwsem_writer_wake() for writers	-- set, clear
- * 3) rwsem_del_waiter()			-- clear
+ * 3) rwsem_del_waiter()		-- clear
  *
  * For all the above cases, wait_lock will be held. A writer must also
  * be the first one in the wait_list to be eligible for setting the handoff
@@ -377,8 +377,8 @@ rwsem_add_waiter(struct rw_semaphore *se
 /*
  * Remove a waiter from the wait_list and clear flags.
  *
- * Both rwsem_mark_wake() and rwsem_writer_wake() contain a full 'copy' of
- * this function. Modify with care.
+ * Both rwsem_{reader,writer}_wake() contain a full 'copy' of this function.
+ * Modify with care.
  *
  * Return: true if wait_list isn't empty and false otherwise
  */
@@ -488,42 +488,15 @@ static void rwsem_writer_wake(struct rw_
 	rwsem_waiter_wake(waiter, wake_q);
 }
 
-/*
- * handle the lock release when processes blocked on it that can now run
- * - if we come here from up_xxxx(), then the RWSEM_FLAG_WAITERS bit must
- *   have been set.
- * - there must be someone on the queue
- * - the wait_lock must be held by the caller
- * - tasks are marked for wakeup, the caller must later invoke wake_up_q()
- *   to actually wakeup the blocked task(s) and drop the reference count,
- *   preferably when the wait_lock is released
- * - woken process blocks are discarded from the list after having task zeroed
- * - writers are only marked woken if downgrading is false
- *
- * Implies rwsem_del_waiter() for all woken readers.
- */
-static void rwsem_mark_wake(struct rw_semaphore *sem,
-			    enum rwsem_wake_type wake_type,
-			    struct wake_q_head *wake_q)
+static void rwsem_reader_wake(struct rw_semaphore *sem,
+			      enum rwsem_wake_type wake_type,
+			      struct rwsem_waiter *waiter,
+			      struct wake_q_head *wake_q)
 {
-	struct rwsem_waiter *waiter, *tmp;
 	long count, woken = 0, adjustment = 0;
+	struct rwsem_waiter *tmp;
 	struct list_head wlist;
 
-	lockdep_assert_held(&sem->wait_lock);
-
-	/*
-	 * Take a peek at the queue head waiter such that we can determine
-	 * the wakeup(s) to perform.
-	 */
-	waiter = rwsem_first_waiter(sem);
-
-	if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
-		if (wake_type == RWSEM_WAKE_ANY)
-			rwsem_writer_wake(sem, waiter, wake_q);
-		return;
-	}
-
 	/*
 	 * No reader wakeup if there are too many of them already.
 	 */
@@ -635,6 +608,42 @@ static void rwsem_mark_wake(struct rw_se
 }
 
 /*
+ * handle the lock release when processes blocked on it that can now run
+ * - if we come here from up_xxxx(), then the RWSEM_FLAG_WAITERS bit must
+ *   have been set.
+ * - there must be someone on the queue
+ * - the wait_lock must be held by the caller
+ * - tasks are marked for wakeup, the caller must later invoke wake_up_q()
+ *   to actually wakeup the blocked task(s) and drop the reference count,
+ *   preferably when the wait_lock is released
+ * - woken process blocks are discarded from the list after having task zeroed
+ * - writers are only marked woken if downgrading is false
+ *
+ * Implies rwsem_del_waiter() for all woken waiters.
+ */
+static void rwsem_mark_wake(struct rw_semaphore *sem,
+			    enum rwsem_wake_type wake_type,
+			    struct wake_q_head *wake_q)
+{
+	struct rwsem_waiter *waiter;
+
+	lockdep_assert_held(&sem->wait_lock);
+
+	/*
+	 * Take a peek at the queue head waiter such that we can determine
+	 * the wakeup(s) to perform.
+	 */
+	waiter = rwsem_first_waiter(sem);
+
+	if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
+		if (wake_type == RWSEM_WAKE_ANY)
+			rwsem_writer_wake(sem, waiter, wake_q);
+	} else {
+		rwsem_reader_wake(sem, wake_type, waiter, wake_q);
+	}
+}
+
+/*
  * Remove a waiter and try to wake up other waiters in the wait queue
  * This function is called from the out_nolock path of both the reader and
  * writer slowpaths with wait_lock held. It releases the wait_lock and
@@ -1017,9 +1026,10 @@ rwsem_down_read_slowpath(struct rw_semap
 		 */
 		if ((rcnt == 1) && (count & RWSEM_FLAG_WAITERS)) {
 			raw_spin_lock_irq(&sem->wait_lock);
-			if (!list_empty(&sem->wait_list))
+			if (!list_empty(&sem->wait_list)) {
 				rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED,
 						&wake_q);
+			}
 			raw_spin_unlock_irq(&sem->wait_lock);
 			wake_up_q(&wake_q);
 		}