perf: Fix inherit vs. context rotation bug

It was found that sometimes children of tasks with inherited events had
one extra event. Eventually it turned out to be due to the list rotation
no being exclusive with the list iteration in the inheritance code.

Cure this by temporarily disabling the rotation while we inherit the events.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Cc: <stable@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 40150f3..142e3d6 100644 (file)
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -850,6 +850,7 @@ struct perf_event_context {
        int                             nr_active;
        int                             is_active;
        int                             nr_stat;
+       int                             rotate_disable;
        atomic_t                        refcount;
        struct task_struct              *task;
 

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 671f6c8..f365dd8 100644 (file)
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -1622,8 +1622,12 @@ static void rotate_ctx(struct perf_event_context *ctx)
 {
        raw_spin_lock(&ctx->lock);
 
-       /* Rotate the first entry last of non-pinned groups */
-       list_rotate_left(&ctx->flexible_groups);
+       /*
+        * Rotate the first entry last of non-pinned groups. Rotation might be
+        * disabled by the inheritance code.
+        */
+       if (!ctx->rotate_disable)
+               list_rotate_left(&ctx->flexible_groups);
 
        raw_spin_unlock(&ctx->lock);
 }
@@ -6162,6 +6166,7 @@ int perf_event_init_context(struct task_struct *child, int ctxn)
        struct perf_event *event;
        struct task_struct *parent = current;
        int inherited_all = 1;
+       unsigned long flags;
        int ret = 0;
 
        child->perf_event_ctxp[ctxn] = NULL;
@@ -6202,6 +6207,15 @@ int perf_event_init_context(struct task_struct *child, int ctxn)
                        break;
        }
 
+       /*
+        * We can't hold ctx->lock when iterating the ->flexible_group list due
+        * to allocations, but we need to prevent rotation because
+        * rotate_ctx() will change the list from interrupt context.
+        */
+       raw_spin_lock_irqsave(&parent_ctx->lock, flags);
+       parent_ctx->rotate_disable = 1;
+       raw_spin_unlock_irqrestore(&parent_ctx->lock, flags);
+
        list_for_each_entry(event, &parent_ctx->flexible_groups, group_entry) {
                ret = inherit_task_group(event, parent, parent_ctx,
                                         child, ctxn, &inherited_all);
@@ -6209,6 +6223,10 @@ int perf_event_init_context(struct task_struct *child, int ctxn)
                        break;
        }
 
+       raw_spin_lock_irqsave(&parent_ctx->lock, flags);
+       parent_ctx->rotate_disable = 0;
+       raw_spin_unlock_irqrestore(&parent_ctx->lock, flags);
+
        child_ctx = child->perf_event_ctxp[ctxn];
 
        if (child_ctx && inherited_all) {