[PATCH] OProfile update

Now we don't do buffer syncs whilst holding current's mmap_sem ever,
we can wait to hold it. Also take task_lock and try to improve the docs a bit.

diff --git a/drivers/oprofile/buffer_sync.c b/drivers/oprofile/buffer_sync.c
index 392fc1729a2747913b0a42bb5cb05d1cfe726c76..a10eecb5a8ad026bf53411ad50e9e436040fd274 100644 (file)
--- a/drivers/oprofile/buffer_sync.c
+++ b/drivers/oprofile/buffer_sync.c
@@ -310,26 +310,23 @@ static void release_mm(struct mm_struct * mm)
 /* Take the task's mmap_sem to protect ourselves from
  * races when we do lookup_dcookie().
  */
-static struct mm_struct * take_task_mm(struct task_struct * task)
+static struct mm_struct * take_tasks_mm(struct task_struct * task)
 {
-       struct mm_struct * mm = task->mm;
- 
-       /* if task->mm !NULL, mm_count must be at least 1. It cannot
-        * drop to 0 without the task exiting, which will have to sleep
-        * on buffer_sem first. So we do not need to mark mm_count
-        * ourselves.
+       struct mm_struct * mm;
+       
+       /* Subtle. We don't need to keep a reference to this task's mm,
+        * because, for the mm to be freed on another CPU, that would have
+        * to go through the task exit notifier, which ends up sleeping
+        * on the buffer_sem we hold, so we end up with mutual exclusion
+        * anyway.
         */
+       task_lock(task);
+       mm = task->mm;
+       task_unlock(task);
+ 
        if (mm) {
-               /* More ugliness. If a task took its mmap
-                * sem then came to sleep on buffer_sem we
-                * will deadlock waiting for it. So we can
-                * but try. This will lose samples :/
-                */
-               if (!down_read_trylock(&mm->mmap_sem)) {
-                       /* FIXME: this underestimates samples lost */
-                       atomic_inc(&oprofile_stats.sample_lost_mmap_sem);
-                       mm = NULL;
-               }
+               /* needed to walk the task's VMAs */
+               down_read(&mm->mmap_sem);
        }
  
        return mm;
@@ -399,7 +396,7 @@ static void sync_buffer(struct oprofile_cpu_buffer * cpu_buf)
                                new = (struct task_struct *)s->event;
 
                                release_mm(mm);
-                               mm = take_task_mm(new);
+                               mm = take_tasks_mm(new);
 
                                cookie = get_exec_dcookie(mm);
                                add_user_ctx_switch(new->pid, cookie);
@@ -460,4 +457,3 @@ static void timer_ping(unsigned long data)
        schedule_work(&sync_wq);
        /* timer is re-added by the scheduled task */
 }
-