perf stat: Avoid sending SIGTERM to random processes

This patch fixes a problem with perf stat whereby on termination it may
send a SIGTERM signal to random processes on systems with high PID
recycling. I got some actual bug reports on this.

There is race between the SIGCHLD and sig_atexit() handlers.  This patch
addresses this problem by clearing child_pid in the SIGCHLD handler.
Signed-off-by: Stephane Eranian <eranian@google.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20130604154426.GA2928@quadSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-stat.c

@@ -87,7 +87,7 @@ static int			run_count			=  1;
 static bool			no_inherit			= false;
 static bool			scale				=  true;
 static enum aggr_mode		aggr_mode			= AGGR_GLOBAL;
-static pid_t			child_pid			= -1;
+static volatile pid_t		child_pid			= -1;
 static bool			null_run			=  false;
 static int			detailed_run			=  0;
 static bool			big_num				=  true;
@@ -1148,13 +1148,34 @@ static void skip_signal(int signo)
 		done = 1;
 
 	signr = signo;
+	/*
+	 * render child_pid harmless
+	 * won't send SIGTERM to a random
+	 * process in case of race condition
+	 * and fast PID recycling
+	 */
+	child_pid = -1;
 }
 
 static void sig_atexit(void)
 {
+	sigset_t set, oset;
+
+	/*
+	 * avoid race condition with SIGCHLD handler
+	 * in skip_signal() which is modifying child_pid
+	 * goal is to avoid send SIGTERM to a random
+	 * process
+	 */
+	sigemptyset(&set);
+	sigaddset(&set, SIGCHLD);
+	sigprocmask(SIG_BLOCK, &set, &oset);
+
 	if (child_pid != -1)
 		kill(child_pid, SIGTERM);
 
+	sigprocmask(SIG_SETMASK, &oset, NULL);
+
 	if (signr == -1)
 		return;