mutex: Use p->on_cpu for the adaptive spin
Peter Zijlstra [Tue, 5 Apr 2011 15:23:41 +0000 (17:23 +0200)]
Since we now have p->on_cpu unconditionally available, use it to
re-implement mutex_spin_on_owner.

Requested-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frank Rowand <frank.rowand@am.sony.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Nick Piggin <npiggin@kernel.dk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20110405152728.826338173@chello.nl

include/linux/mutex.h
include/linux/sched.h
kernel/mutex-debug.c
kernel/mutex-debug.h
kernel/mutex.c
kernel/mutex.h
kernel/sched.c

index 94b48bd..c75471d 100644 (file)
@@ -51,7 +51,7 @@ struct mutex {
        spinlock_t              wait_lock;
        struct list_head        wait_list;
 #if defined(CONFIG_DEBUG_MUTEXES) || defined(CONFIG_SMP)
-       struct thread_info      *owner;
+       struct task_struct      *owner;
 #endif
 #ifdef CONFIG_DEBUG_MUTEXES
        const char              *name;
index 3435837..1738504 100644 (file)
@@ -360,7 +360,7 @@ extern signed long schedule_timeout_interruptible(signed long timeout);
 extern signed long schedule_timeout_killable(signed long timeout);
 extern signed long schedule_timeout_uninterruptible(signed long timeout);
 asmlinkage void schedule(void);
-extern int mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner);
+extern int mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner);
 
 struct nsproxy;
 struct user_namespace;
index ec815a9..73da83a 100644 (file)
@@ -75,7 +75,7 @@ void debug_mutex_unlock(struct mutex *lock)
                return;
 
        DEBUG_LOCKS_WARN_ON(lock->magic != lock);
-       DEBUG_LOCKS_WARN_ON(lock->owner != current_thread_info());
+       DEBUG_LOCKS_WARN_ON(lock->owner != current);
        DEBUG_LOCKS_WARN_ON(!lock->wait_list.prev && !lock->wait_list.next);
        mutex_clear_owner(lock);
 }
index 57d527a..0799fd3 100644 (file)
@@ -29,7 +29,7 @@ extern void debug_mutex_init(struct mutex *lock, const char *name,
 
 static inline void mutex_set_owner(struct mutex *lock)
 {
-       lock->owner = current_thread_info();
+       lock->owner = current;
 }
 
 static inline void mutex_clear_owner(struct mutex *lock)
index c4195fa..fe4706c 100644 (file)
@@ -160,7 +160,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
         */
 
        for (;;) {
-               struct thread_info *owner;
+               struct task_struct *owner;
 
                /*
                 * If we own the BKL, then don't spin. The owner of
index 67578ca..4115fbf 100644 (file)
@@ -19,7 +19,7 @@
 #ifdef CONFIG_SMP
 static inline void mutex_set_owner(struct mutex *lock)
 {
-       lock->owner = current_thread_info();
+       lock->owner = current;
 }
 
 static inline void mutex_clear_owner(struct mutex *lock)
index cd2593e..55cc503 100644 (file)
@@ -4173,70 +4173,53 @@ need_resched:
 EXPORT_SYMBOL(schedule);
 
 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
-/*
- * Look out! "owner" is an entirely speculative pointer
- * access and not reliable.
- */
-int mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner)
-{
-       unsigned int cpu;
-       struct rq *rq;
 
-       if (!sched_feat(OWNER_SPIN))
-               return 0;
+static inline bool owner_running(struct mutex *lock, struct task_struct *owner)
+{
+       bool ret = false;
 
-#ifdef CONFIG_DEBUG_PAGEALLOC
-       /*
-        * Need to access the cpu field knowing that
-        * DEBUG_PAGEALLOC could have unmapped it if
-        * the mutex owner just released it and exited.
-        */
-       if (probe_kernel_address(&owner->cpu, cpu))
-               return 0;
-#else
-       cpu = owner->cpu;
-#endif
+       rcu_read_lock();
+       if (lock->owner != owner)
+               goto fail;
 
        /*
-        * Even if the access succeeded (likely case),
-        * the cpu field may no longer be valid.
+        * Ensure we emit the owner->on_cpu, dereference _after_ checking
+        * lock->owner still matches owner, if that fails, owner might
+        * point to free()d memory, if it still matches, the rcu_read_lock()
+        * ensures the memory stays valid.
         */
-       if (cpu >= nr_cpumask_bits)
-               return 0;
+       barrier();
 
-       /*
-        * We need to validate that we can do a
-        * get_cpu() and that we have the percpu area.
-        */
-       if (!cpu_online(cpu))
-               return 0;
+       ret = owner->on_cpu;
+fail:
+       rcu_read_unlock();
 
-       rq = cpu_rq(cpu);
+       return ret;
+}
 
-       for (;;) {
-               /*
-                * Owner changed, break to re-assess state.
-                */
-               if (lock->owner != owner) {
-                       /*
-                        * If the lock has switched to a different owner,
-                        * we likely have heavy contention. Return 0 to quit
-                        * optimistic spinning and not contend further:
-                        */
-                       if (lock->owner)
-                               return 0;
-                       break;
-               }
+/*
+ * Look out! "owner" is an entirely speculative pointer
+ * access and not reliable.
+ */
+int mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
+{
+       if (!sched_feat(OWNER_SPIN))
+               return 0;
 
-               /*
-                * Is that owner really running on that cpu?
-                */
-               if (task_thread_info(rq->curr) != owner || need_resched())
+       while (owner_running(lock, owner)) {
+               if (need_resched())
                        return 0;
 
                arch_mutex_cpu_relax();
        }
 
+       /*
+        * If the owner changed to another task there is likely
+        * heavy contention, stop spinning.
+        */
+       if (lock->owner)
+               return 0;
+
        return 1;
 }
 #endif