perf: Fix NULL deref in inheritance code

Liming found a NULL deref when a task has a perf context but no
counters  when it forks.

This can occur in two cases, a race during construction where
the fork hits after installing the context but before the first
counter gets inserted, or more reproducably, a fork after the
last counter is closed (which leaves the context around).
Reported-by: Wang Liming <liming.wang@windriver.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
CC: <stable@kernel.org>
LKML-Reference: <1262185684.7135.222.camel@laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/perf_event.c

@@ -5148,7 +5148,7 @@ int perf_event_init_task(struct task_struct *child)
 					    GFP_KERNEL);
 			if (!child_ctx) {
 				ret = -ENOMEM;
-				goto exit;
+				break;
 			}
 
 			__perf_event_init_context(child_ctx, child);
@@ -5164,7 +5164,7 @@ int perf_event_init_task(struct task_struct *child)
 		}
 	}
 
-	if (inherited_all) {
+	if (child_ctx && inherited_all) {
 		/*
 		 * Mark the child context as a clone of the parent
 		 * context, or of whatever the parent is a clone of.
@@ -5184,7 +5184,6 @@ int perf_event_init_task(struct task_struct *child)
 		get_ctx(child_ctx->parent_ctx);
 	}
 
-exit:
 	mutex_unlock(&parent_ctx->mutex);
 
 	perf_unpin_context(parent_ctx);