perf: Identify the 0->1 transition for event::mmap_count

Needed because refcount_inc() doesn't allow the 0->1 transition.

Specifically, this is the case where we've created the RB, this means
there was no RB, and as such there could not have been an mmap.
Additionally we hold mmap_mutex to serialize everything.

This must be the first.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250812104019.956479989@infradead.org

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 41941dfadfcbc756233e5c28f5f620f15e3615b1..f6211ab185036426303f9c20214f26147cb78afa 100644 (file)
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7034,7 +7034,7 @@ static int perf_mmap_rb(struct vm_area_struct *vma, struct perf_event *event,
        perf_event_update_userpage(event);
 
        perf_mmap_account(vma, user_extra, extra);
-       atomic_inc(&event->mmap_count);
+       atomic_set(&event->mmap_count, 1);
 
        return 0;
 }