oprofile: warn on freeing event buffer too early

A race shouldn't happen since all workqueues or handlers are canceled
or flushed before the event buffer is freed. A warning is triggered
now if the buffer is freed too early.

Also, this patch adds some comments about event buffer protection,
reworks some code and adds code to clear buffer_pos during alloc and
free of the event buffer.

Cc: David Rientjes <rientjes@google.com>
Cc: Stephane Eranian <eranian@google.com>
Signed-off-by: Robert Richter <robert.richter@amd.com>

diff --git a/drivers/oprofile/event_buffer.c b/drivers/oprofile/event_buffer.c
index c38adb3..5df60a6 100644 (file)
--- a/drivers/oprofile/event_buffer.c
+++ b/drivers/oprofile/event_buffer.c
@@ -35,17 +35,22 @@ static size_t buffer_pos;
 /* atomic_t because wait_event checks it outside of buffer_mutex */
 static atomic_t buffer_ready = ATOMIC_INIT(0);
 
-/* Add an entry to the event buffer. When we
- * get near to the end we wake up the process
- * sleeping on the read() of the file.
+/*
+ * Add an entry to the event buffer. When we get near to the end we
+ * wake up the process sleeping on the read() of the file. To protect
+ * the event_buffer this function may only be called when buffer_mutex
+ * is set.
  */
 void add_event_entry(unsigned long value)
 {
        /*
-        * catch potential error
+        * This shouldn't happen since all workqueues or handlers are
+        * canceled or flushed before the event buffer is freed.
         */
-       if (!event_buffer)
+       if (!event_buffer) {
+               WARN_ON_ONCE(1);
                return;
+       }
 
        if (buffer_pos == buffer_size) {
                atomic_inc(&oprofile_stats.event_lost_overflow);
@@ -75,7 +80,6 @@ void wake_up_buffer_waiter(void)
 
 int alloc_event_buffer(void)
 {
-       int err = -ENOMEM;
        unsigned long flags;
 
        spin_lock_irqsave(&oprofilefs_lock, flags);
@@ -86,13 +90,12 @@ int alloc_event_buffer(void)
        if (buffer_watershed >= buffer_size)
                return -EINVAL;
 
+       buffer_pos = 0;
        event_buffer = vmalloc(sizeof(unsigned long) * buffer_size);
        if (!event_buffer)
-               goto out;
+               return -ENOMEM;
 
-       err = 0;
-out:
-       return err;
+       return 0;
 }
 
 
@@ -100,6 +103,7 @@ void free_event_buffer(void)
 {
        mutex_lock(&buffer_mutex);
        vfree(event_buffer);
+       buffer_pos = 0;
        event_buffer = NULL;
        mutex_unlock(&buffer_mutex);
 }
@@ -174,6 +178,7 @@ static ssize_t event_buffer_read(struct file *file, char __user *buf,
 
        mutex_lock(&buffer_mutex);
 
+       /* May happen if the buffer is freed during pending reads. */
        if (!event_buffer) {
                retval = -EINTR;
                goto out;