perf sched stats: Fix SIGCHLD vs pause() race in schedstat_record()

If the profiled workload exits very quickly, SIGCHLD can be delivered
and consumed by the empty signal handler before the process enters
pause(), causing an indefinite hang.

Fix this with a simpler approach:

 - The signal handler now sets a 'volatile sig_atomic_t done' flag.
   Reset 'done' before registering signal handlers so that an early
   signal during setup is not discarded by a later reset.

 - Replace pause() with a loop that checks 'done' and uses
   waitpid(WNOHANG) to detect child exit without blocking.  This
   handles both workload mode (child exits) and system-wide mode
   (user sends SIGINT/SIGTERM).  Using WNOHANG avoids the SA_RESTART
   problem where a blocking waitpid() would auto-restart and ignore
   the done flag if the child doesn't exit on signal.

Suggested-by: Namhyung Kim <namhyung@kernel.org>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Swapnil Sapkal <swapnil.sapkal@amd.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Bangoria <ravi.bangoria@amd.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

diff --git a/tools/perf/builtin-sched.c b/tools/perf/builtin-sched.c
index 9d73c704318294f5926a8e391a0af9825678e8a4..4f54a08e66728ee9cb9006e9d0ca039df472cd0c 100644 (file)
--- a/tools/perf/builtin-sched.c
+++ b/tools/perf/builtin-sched.c
@@ -36,6 +36,7 @@
 #include <linux/zalloc.h>
 #include <sys/prctl.h>
 #include <sys/resource.h>
+#include <sys/wait.h>
 #include <inttypes.h>
 
 #include <errno.h>
@@ -3746,8 +3747,11 @@ static int process_synthesized_schedstat_event(const struct perf_tool *tool,
        return 0;
 }
 
+static volatile sig_atomic_t done;
+
 static void sighandler(int sig __maybe_unused)
 {
+       done = 1;
 }
 
 static int enable_sched_schedstats(int *reset)
@@ -3807,6 +3811,7 @@ static int perf_sched__schedstat_record(struct perf_sched *sched,
                .mode  = PERF_DATA_MODE_WRITE,
        };
 
+       done = 0;
        signal(SIGINT, sighandler);
        signal(SIGCHLD, sighandler);
        signal(SIGTERM, sighandler);
@@ -3891,8 +3896,11 @@ static int perf_sched__schedstat_record(struct perf_sched *sched,
        if (argc)
                evlist__start_workload(evlist);
 
-       /* wait for signal */
-       pause();
+       while (!done) {
+               if (argc && waitpid(evlist->workload.pid, NULL, WNOHANG) > 0)
+                       break;
+               sleep(1);
+       }
 
        if (reset) {
                err = disable_sched_schedstat();