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    Subject[PATCH] perf record: Use an eventfd to wakeup when done
    From
    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@google.com>

    ---

    tools/perf/builtin-record.c | 8 ++++++++
    1 file changed, 8 insertions(+)

    diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
    index 1ab349abe90469..ce5fc3860131d2 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 +519,19 @@ 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;
    + 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);
    +
    atexit(record__sig_exit);
    signal(SIGCHLD, sig_handler);
    signal(SIGINT, sig_handler);
    --
    2.26.2.645.ge9eca65c58-goog
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