locking/mutex: Refactor mutex_spin_on_owner()

This patch applies on top of tip.

-------------------------------------------------------------------
Similar to what Linus suggested for rwsem_spin_on_owner(), in mutex_spin_on_owner(), instead of having while (true) and breaking out of the spin loop on lock->owner != owner, we can have the loop directly check for while (lock->owner == owner). This improves the readability of the code.

Signed-off-by Jason Low <[email protected]>

---
 kernel/locking/mutex.c |   17 +++++------------
 1 files changed, 5 insertions(+), 12 deletions(-)

kernel/locking/mutex.c +2 -11

index 16b2d3c..1c3b7c5 100644
@@ -224,16 +224,8 @@ ww_mutex_set_context_slowpath(struct ww_mutex *lock,
 static noinline
 bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
 {
-	bool ret;
-
 	rcu_read_lock();
-	while (true) {
-		/* Return success when the lock owner changed */
-		if (lock->owner != owner) {
-			ret = true;
-			break;
-		}
-
+	while (lock->owner == owner) {
 		/*
 		 * Ensure we emit the owner->on_cpu, dereference _after_
 		 * checking lock->owner still matches owner, if that fails,

@@ -242,16 +234,17 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
 		 */
 		barrier();
 
+		/* Stop spinning when need_resched or owner is not running. */
 		if (!owner->on_cpu || need_resched()) {
-			ret = false;
-			break;

Quoted 52 lines · Jason Low

This patch applies on top of tip.

-------------------------------------------------------------------
Similar to what Linus suggested for rwsem_spin_on_owner(), in mutex_spin_on_owner(), instead of having while (true) and breaking out of the spin loop on lock->owner != owner, we can have the loop directly check for while (lock->owner == owner). This improves the readability of the code.

Signed-off-by Jason Low <[email protected]>

---
 kernel/locking/mutex.c |   17 +++++------------
 1 files changed, 5 insertions(+), 12 deletions(-)

kernel/locking/mutex.c +4 -11

index 16b2d3c..1c3b7c5 100644
@@ -224,16 +224,8 @@ ww_mutex_set_context_slowpath(struct ww_mutex *lock,
 static noinline
 bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
 {
-	bool ret;
-
 	rcu_read_lock();
-	while (true) {
-		/* Return success when the lock owner changed */
-		if (lock->owner != owner) {
-			ret = true;
-			break;
-		}
-
+	while (lock->owner == owner) {
 		/*
 		 * Ensure we emit the owner->on_cpu, dereference _after_
 		 * checking lock->owner still matches owner, if that fails,

@@ -242,16 +234,17 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
 		 */
 		barrier();
 
+		/* Stop spinning when need_resched or owner is not running. */
 		if (!owner->on_cpu || need_resched()) {
-			ret = false;
-			break;
+			rcu_read_unlock();
+			return false;
 		}
 
 		cpu_relax_lowlatency();
 	}
 	rcu_read_unlock();
 

kernel/locking/mutex.c bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner) · line 247

Quoted 2 lines

kernel/locking/mutex.c +1 -1

-	return ret;
+	return true;

A nit: having multiple return statements in a function is not the cleanest approach, especially when we are holding locks.

It's better to add an 'out_unlock' label to before the rcu_read_unlock() and use that plus 'ret'.

Quoted 60 lines · Ingo Molnar

Quoted 44 lines · Jason Low

This patch applies on top of tip.

-------------------------------------------------------------------
Similar to what Linus suggested for rwsem_spin_on_owner(), in mutex_spin_on_owner(), instead of having while (true) and breaking out of the spin loop on lock->owner != owner, we can have the loop directly check for while (lock->owner == owner). This improves the readability of the code.

Signed-off-by Jason Low <[email protected]>

---
 kernel/locking/mutex.c |   17 +++++------------
 1 files changed, 5 insertions(+), 12 deletions(-)

kernel/locking/mutex.c +2 -11

index 16b2d3c..1c3b7c5 100644
@@ -224,16 +224,8 @@ ww_mutex_set_context_slowpath(struct ww_mutex *lock,
 static noinline
 bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
 {
-	bool ret;
-
 	rcu_read_lock();
-	while (true) {
-		/* Return success when the lock owner changed */
-		if (lock->owner != owner) {
-			ret = true;
-			break;
-		}
-
+	while (lock->owner == owner) {
 		/*
 		 * Ensure we emit the owner->on_cpu, dereference _after_
 		 * checking lock->owner still matches owner, if that fails,

@@ -242,16 +234,17 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
 		 */
 		barrier();
 
+		/* Stop spinning when need_resched or owner is not running. */
 		if (!owner->on_cpu || need_resched()) {
-			ret = false;
-			break;

kernel/locking/mutex.c bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner) · line 247

Quoted 10 lines

kernel/locking/mutex.c +3 -1

+			rcu_read_unlock();
+			return false;
 		}
 
 		cpu_relax_lowlatency();
 	}
 	rcu_read_unlock();
 
-	return ret;
+	return true;

A nit: having multiple return statements in a function is not the cleanest approach, especially when we are holding locks.

Quoted 2 lines

It's better to add an 'out_unlock' label to before the rcu_read_unlock() and use that plus 'ret'.

Okay, I can update this patch. Should we make another similar update for the rwsem then?