perf, core: Fix initial task_ctx/event installation

A lost Quilt refresh of 2c29ef0f (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@laptopSigned-off-by: Ingo Molnar <mingo@elte.hu>

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);