perf/core: Remove retry loop from perf_mmap()

AFAICT there is no actual benefit from the mutex drop on re-try. The
'worst' case scenario is that we instantly re-gain the mutex without
perf_mmap_close() getting it. So might as well make that the normal
case.

Reflow the code to make the ring buffer detach case naturally flow
into the no ring buffer case.

[ mingo: Forward ported it ]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Ravi Bangoria <ravi.bangoria@amd.com>
Link: https://lore.kernel.org/r/20241104135519.463607258@infradead.org

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 4cd3494c65e20ce0a2081bd5374ffaa93b327c14..ca4c1242c29b50bb0249439ec08f765a1eecefd2 100644 (file)
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6719,28 +6719,33 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
                        return -EINVAL;
 
                WARN_ON_ONCE(event->ctx->parent_ctx);
-again:
                mutex_lock(&event->mmap_mutex);
+
                if (event->rb) {
                        if (data_page_nr(event->rb) != nr_pages) {
                                ret = -EINVAL;
                                goto unlock;
                        }
 
-                       if (!atomic_inc_not_zero(&event->rb->mmap_count)) {
+                       if (atomic_inc_not_zero(&event->rb->mmap_count)) {
                                /*
-                                * Raced against perf_mmap_close(); remove the
-                                * event and try again.
+                                * Success -- managed to mmap() the same buffer
+                                * multiple times.
                                 */
-                               ring_buffer_attach(event, NULL);
-                               mutex_unlock(&event->mmap_mutex);
-                               goto again;
+                               ret = 0;
+                               /* We need the rb to map pages. */
+                               rb = event->rb;
+                               goto unlock;
                        }
 
-                       /* We need the rb to map pages. */
-                       rb = event->rb;
-                       goto unlock;
+                       /*
+                        * Raced against perf_mmap_close()'s
+                        * atomic_dec_and_mutex_lock() remove the
+                        * event and continue as if !event->rb
+                        */
+                       ring_buffer_attach(event, NULL);
                }
+
        } else {
                /*
                 * AUX area mapping: if rb->aux_nr_pages != 0, it's already