perf tools: Don't stop synthesizing threads when one vanishes

The perf_event__synthesize_threads routine synthesizes all the existing
threads in the system, because we don't have any kernel facilities to
ask for PERF_RECORD_{FORK,MMAP,COMM} for existing threads.

It was returning an error as soon as one thread couldn't be synthesized,
which is a bit extreme when, for instance, a forkish workload is
running, like a kernel compile.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-i7oas1eodpoer2bx38fwyasv@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/event.c

@@ -405,16 +405,15 @@ int perf_event__synthesize_threads(struct perf_tool *tool,
 
 		if (*end) /* only interested in proper numerical dirents */
 			continue;
-
-		if (__event__synthesize_thread(comm_event, mmap_event, pid, 1,
-					   process, tool, machine) != 0) {
-			err = -1;
-			goto out_closedir;
-		}
+		/*
+ 		 * We may race with exiting thread, so don't stop just because
+ 		 * one thread couldn't be synthesized.
+ 		 */
+		__event__synthesize_thread(comm_event, mmap_event, pid, 1,
+					   process, tool, machine);
 	}
 
 	err = 0;
-out_closedir:
 	closedir(proc);
 out_free_mmap:
 	free(mmap_event);