perf_counter: fix counter inheritance race

Context rotation should not occur when we are in the middle of
walking the counter list when inheriting counters ...

[ Impact: fix occasionally incorrect perf stat results ]

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 10 insertions, 1 deletions

diff --git a/include/linux/perf_counter.h b/include/linux/perf_counter.h
index c8c1dfc22c93..13cb2fbbf334 100644
--- a/include/linux/perf_counter.h
+++ b/include/linux/perf_counter.h
@@ -508,6 +508,7 @@ struct perf_counter_context {
         int                     nr_counters;
         int                     nr_active;
         int                     is_active;
+        int                     rr_allowed;
         struct task_struct      *task;
 
         /*
diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
index 7af16d1c480f..4d8f97375f3a 100644
--- a/kernel/perf_counter.c
+++ b/kernel/perf_counter.c
@@ -1120,7 +1120,8 @@ void perf_counter_task_tick(struct task_struct *curr, int cpu)
         __perf_counter_task_sched_out(ctx);
 
         rotate_ctx(&cpuctx->ctx);
-        rotate_ctx(ctx);
+        if (ctx->rr_allowed)
+                rotate_ctx(ctx);
 
         perf_counter_cpu_sched_in(cpuctx, cpu);
         perf_counter_task_sched_in(curr, cpu);
@@ -3108,6 +3109,7 @@ __perf_counter_init_context(struct perf_counter_context *ctx,
         mutex_init(&ctx->mutex);
         INIT_LIST_HEAD(&ctx->counter_list);
         INIT_LIST_HEAD(&ctx->event_list);
+        ctx->rr_allowed = 1;
         ctx->task = task;
 }
 
@@ -3348,6 +3350,9 @@ void perf_counter_init_task(struct task_struct *child)
          */
         mutex_lock(&parent_ctx->mutex);
 
+        parent_ctx->rr_allowed = 0;
+        barrier(); /* irqs */
+
         /*
          * We dont have to disable NMIs - we are only looking at
          * the list, not manipulating it:
@@ -3361,6 +3366,9 @@ void perf_counter_init_task(struct task_struct *child)
                         break;
         }
 
+        barrier(); /* irqs */
+        parent_ctx->rr_allowed = 1;
+
         mutex_unlock(&parent_ctx->mutex);
 }