perf, core: Fix initial task_ctx/event installation

A lost Quilt refresh of 2c29ef0fef8 (perf: Simplify and fix
__perf_install_in_context()) is causing grief and lockups,
reported by Jiri Olsa.

When installing an event in a task context, there's a number of
issues:

 - there might not be an existing task context, in which case
   we should install the now current context;

 - there might already be a context, not the current one, in
   which case we should de-schedule the old and install the new;

these cases were dealt with in the lost refresh, however there is one
further case that was found in testing:

 - there might already be a context, the current one, in which
   case we should still de-schedule, and should take care
   to re-install it (note that task_ctx_sched_out() clears
   cpuctx->task_ctx).

Reported-by: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1307399008.2497.971.camel@laptop
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 17 insertions, 11 deletions

diff --git a/kernel/events/core.c b/kernel/events/core.c
index ba89f40abe6..5e8c7b1389b 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1505,25 +1505,31 @@ static int  __perf_install_in_context(void *info)
         struct perf_event_context *task_ctx = cpuctx->task_ctx;
         struct task_struct *task = current;
 
-        perf_ctx_lock(cpuctx, cpuctx->task_ctx);
+        perf_ctx_lock(cpuctx, task_ctx);
         perf_pmu_disable(cpuctx->ctx.pmu);
 
         /*
          * If there was an active task_ctx schedule it out.
          */
-        if (task_ctx) {
+        if (task_ctx)
                 task_ctx_sched_out(task_ctx);
-                /*
-                 * If the context we're installing events in is not the
-                 * active task_ctx, flip them.
-                 */
-                if (ctx->task && task_ctx != ctx) {
-                        raw_spin_unlock(&cpuctx->ctx.lock);
-                        raw_spin_lock(&ctx->lock);
-                        cpuctx->task_ctx = task_ctx = ctx;
-                }
+
+        /*
+         * If the context we're installing events in is not the
+         * active task_ctx, flip them.
+         */
+        if (ctx->task && task_ctx != ctx) {
+                if (task_ctx)
+                        raw_spin_unlock(&task_ctx->lock);
+                raw_spin_lock(&ctx->lock);
+                task_ctx = ctx;
+        }
+
+        if (task_ctx) {
+                cpuctx->task_ctx = task_ctx;
                 task = task_ctx->task;
         }
+
         cpu_ctx_sched_out(cpuctx, EVENT_ALL);
 
         update_context_time(ctx);