Re: [PATCH] locking/mutex: clear MUTEX_FLAGS if wait_list is empty due to signal

[Peter Zijlstra <peterz@infradead.org>]

On Sat, May 15, 2021 at 10:30:10AM +0800, qiang.zhang@windriver.com wrote:
> From: Zqiang <qiang.zhang@windriver.com>
> 
> Clear MUTEX_FLAGS when call mutex_lock_interruptible()
> interrupted by a signal and the lock->wait_list is empty.

That's what the patches does; and I can read the C code perfectly fine,
thank you. What the C code doesn't tell me is why, nor how you came to
write this patch, was there an actual problem that is solved? Were you
bored and just reading the code?

That is, your Changelog tells me absolutely nothing.

> Signed-off-by: Zqiang <qiang.zhang@windriver.com>
> ---
>  kernel/locking/mutex.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index cb6b112ce155..4ac354ca092b 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -1081,6 +1081,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>  err:
>  	__set_current_state(TASK_RUNNING);
>  	mutex_remove_waiter(lock, &waiter, current);
> +	if (likely(list_empty(&lock->wait_list)))
> +		__mutex_clear_flag(lock, MUTEX_FLAGS);

Would not the nicer patch be something like this?

---
 kernel/locking/mutex-debug.c |  6 +++---
 kernel/locking/mutex-debug.h |  4 ++--
 kernel/locking/mutex.c       | 17 ++++++++++++-----
 kernel/locking/mutex.h       |  4 +---
 4 files changed, 18 insertions(+), 13 deletions(-)

diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c
index a7276aaf2abc..447013815200 100644
--- a/kernel/locking/mutex-debug.c
+++ b/kernel/locking/mutex-debug.c
@@ -57,15 +57,15 @@ void debug_mutex_add_waiter(struct mutex *lock, struct mutex_waiter *waiter,
 	task->blocked_on = waiter;
 }
 
-void mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter,
-			 struct task_struct *task)
+void debug_mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter,
+			       struct task_struct *task)
 {
 	DEBUG_LOCKS_WARN_ON(list_empty(&waiter->list));
 	DEBUG_LOCKS_WARN_ON(waiter->task != task);
 	DEBUG_LOCKS_WARN_ON(task->blocked_on != waiter);
 	task->blocked_on = NULL;
 
-	list_del_init(&waiter->list);
+	INIT_LIST_HEAD(&waiter->list);
 	waiter->task = NULL;
 }
 
diff --git a/kernel/locking/mutex-debug.h b/kernel/locking/mutex-debug.h
index 1edd3f45a4ec..e50afe4cc871 100644
--- a/kernel/locking/mutex-debug.h
+++ b/kernel/locking/mutex-debug.h
@@ -22,8 +22,8 @@ extern void debug_mutex_free_waiter(struct mutex_waiter *waiter);
 extern void debug_mutex_add_waiter(struct mutex *lock,
 				   struct mutex_waiter *waiter,
 				   struct task_struct *task);
-extern void mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter,
-				struct task_struct *task);
+extern void debug_mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter,
+				      struct task_struct *task);
 extern void debug_mutex_unlock(struct mutex *lock);
 extern void debug_mutex_init(struct mutex *lock, const char *name,
 			     struct lock_class_key *key);
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index cb6b112ce155..5598920b49b0 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -205,6 +205,16 @@ __mutex_add_waiter(struct mutex *lock, struct mutex_waiter *waiter,
 		__mutex_set_flag(lock, MUTEX_FLAG_WAITERS);
 }
 
+static void
+__mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter)
+{
+	__list_del(waiter->list.prev, waiter->list.next);
+	debug_mutex_remove_waiter(lock, &waiter, current);
+	if (likely(list_empty(&lock->wait_list)))
+		__mutex_clear_flag(lock, MUTEX_FLAGS);
+
+}
+
 /*
  * Give up ownership to a specific task, when @task = NULL, this is equivalent
  * to a regular unlock. Sets PICKUP on a handoff, clears HANDOFF, preserves
@@ -1061,10 +1071,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 			__ww_mutex_check_waiters(lock, ww_ctx);
 	}
 
-	mutex_remove_waiter(lock, &waiter, current);
-	if (likely(list_empty(&lock->wait_list)))
-		__mutex_clear_flag(lock, MUTEX_FLAGS);
-
+	__mutex_remove_waiter(lock, &waiter);
 	debug_mutex_free_waiter(&waiter);
 
 skip_wait:
@@ -1080,7 +1087,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 
 err:
 	__set_current_state(TASK_RUNNING);
-	mutex_remove_waiter(lock, &waiter, current);
+	__mutex_remove_waiter(lock, &waiter);
 err_early_kill:
 	spin_unlock(&lock->wait_lock);
 	debug_mutex_free_waiter(&waiter);
diff --git a/kernel/locking/mutex.h b/kernel/locking/mutex.h
index 1c2287d3fa71..1d4ddb415c22 100644
--- a/kernel/locking/mutex.h
+++ b/kernel/locking/mutex.h
@@ -10,12 +10,10 @@
  * !CONFIG_DEBUG_MUTEXES case. Most of them are NOPs:
  */
 
-#define mutex_remove_waiter(lock, waiter, task) \
-		__list_del((waiter)->list.prev, (waiter)->list.next)
-
 #define debug_mutex_wake_waiter(lock, waiter)		do { } while (0)
 #define debug_mutex_free_waiter(waiter)			do { } while (0)
 #define debug_mutex_add_waiter(lock, waiter, ti)	do { } while (0)
+#define debug_mutex_remove_waiter(lock, waiter, ti)	do { } while (0)
 #define debug_mutex_unlock(lock)			do { } while (0)
 #define debug_mutex_init(lock, name, key)		do { } while (0)