Re: [PATCH] perf record: Use an eventfd to wakeup when done

Jiri Olsa
2020-05-11T11:28:47Z

On Fri, May 08, 2020 at 02:56:43PM +1000, Anand K Mistry wrote:
> 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.

so it's just few instructions in between the check and the evlist__poll

if (done || draining)
break;
err = evlist__poll(rec->evlist, -1);

nice catch!

SNIP

> @@ -518,15 +519,19 @@ 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;

could you please put some explaining comment in here,
so we are not confused by this in few months ;-)

> + if (write(done_fd, &tmp, sizeof(tmp)) < 0)
> + pr_err("failed to signal wakeup fd\n");
> }
>
> static void sigsegv_handler(int sig)
> @@ -1424,6 +1429,9 @@ static int __cmd_record(struct record *rec, int argc, const char **argv)
> int fd;
> float ratio = 0;
>
> + done_fd = eventfd(0, EFD_NONBLOCK);
> + evlist__add_pollfd(rec->evlist, done_fd);

both of those can fail, please check the return values

thanks,
jirka

> +
> atexit(record__sig_exit);
> signal(SIGCHLD, sig_handler);
> signal(SIGINT, sig_handler);
> --
> 2.26.2.645.ge9eca65c58-goog
>