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Date: Tue, 12 May 2020 14:59:36 +1000 From: Anand K Mistry <amistry@...gle.com> To: linux-perf-users@...r.kernel.org Cc: Anand K Mistry <amistry@...gle.com>, Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Arnaldo Carvalho de Melo <acme@...nel.org>, Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...hat.com>, Mark Rutland <mark.rutland@....com>, Namhyung Kim <namhyung@...nel.org>, Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org Subject: [PATCH] perf record: Use an eventfd to wakeup when done The setting and checking of 'done' contains a rare race where the signal handler setting 'done' is run after checking to break the loop, but before waiting in evlist__poll(). In this case, the main loop won't wake up until either another signal is sent, or the perf data fd causes a wake up. The following simple script can trigger this condition (but you might need to run it for several hours): for ((i = 0; i >= 0; i++)) ; do echo "Loop $i" delay=$(echo "scale=4; 0.1 * $RANDOM/32768" | bc) ./perf record -- sleep 30000000 >/dev/null& pid=$! sleep $delay kill -TERM $pid echo "PID $pid" wait $pid done At some point, the loop will stall. Adding logging, even though perf has received the SIGTERM and set 'done = 1', perf will remain sleeping until a second signal is sent. Signed-off-by: Anand K Mistry <amistry@...gle.com> --- tools/perf/builtin-record.c | 25 +++++++++++++++++++++++++ 1 file changed, 25 insertions(+) diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c index 1ab349abe90469..099ecaa66732a2 100644 --- a/tools/perf/builtin-record.c +++ b/tools/perf/builtin-record.c @@ -53,6 +53,7 @@ #include <unistd.h> #include <sched.h> #include <signal.h> +#include <sys/eventfd.h> #include <sys/mman.h> #include <sys/wait.h> #include <sys/types.h> @@ -518,15 +519,28 @@ static int record__pushfn(struct mmap *map, void *to, void *bf, size_t size) static volatile int signr = -1; static volatile int child_finished; +static int done_fd = -1; static void sig_handler(int sig) { + u64 tmp = 1; if (sig == SIGCHLD) child_finished = 1; else signr = sig; done = 1; + + /* + * It is possible for this signal handler to run after done is checked + * in the main loop, but before the perf counter fds are polled. If this + * happens, the poll() will continue to wait even though done is set, + * and will only break out if either another signal is received, or the + * counters are ready for read. To ensure the poll() doesn't sleep when + * done is set, use an eventfd (done_fd) to wake up the poll(). + */ + if (write(done_fd, &tmp, sizeof(tmp)) < 0) + pr_err("failed to signal wakeup fd\n"); } static void sigsegv_handler(int sig) @@ -1424,6 +1438,17 @@ static int __cmd_record(struct record *rec, int argc, const char **argv) int fd; float ratio = 0; + done_fd = eventfd(0, EFD_NONBLOCK); + if (done_fd < 0) { + pr_err("Failed to create wakeup eventfd, error: %m\n"); + return -1; + } + err = evlist__add_pollfd(rec->evlist, done_fd); + if (err < 0) { + pr_err("Failed to add wakeup eventfd to poll list\n"); + return -1; + } + atexit(record__sig_exit); signal(SIGCHLD, sig_handler); signal(SIGINT, sig_handler); -- 2.26.2.645.ge9eca65c58-goog
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