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    Subject[PATCH AUTOSEL 5.4 087/175] perf: Add cond_resched() to task_function_call()
    Date
    From: Barret Rhoden <brho@google.com>

    [ Upstream commit 2ed6edd33a214bca02bd2b45e3fc3038a059436b ]

    Under rare circumstances, task_function_call() can repeatedly fail and
    cause a soft lockup.

    There is a slight race where the process is no longer running on the cpu
    we targeted by the time remote_function() runs. The code will simply
    try again. If we are very unlucky, this will continue to fail, until a
    watchdog fires. This can happen in a heavily loaded, multi-core virtual
    machine.

    Reported-by: syzbot+bb4935a5c09b5ff79940@syzkaller.appspotmail.com
    Signed-off-by: Barret Rhoden <brho@google.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Link: https://lkml.kernel.org/r/20200414222920.121401-1-brho@google.com
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/events/core.c | 23 ++++++++++++++---------
    1 file changed, 14 insertions(+), 9 deletions(-)

    diff --git a/kernel/events/core.c b/kernel/events/core.c
    index 7382fc95d41e..aaaf50b25cc9 100644
    --- a/kernel/events/core.c
    +++ b/kernel/events/core.c
    @@ -93,11 +93,11 @@ static void remote_function(void *data)
    * @info: the function call argument
    *
    * Calls the function @func when the task is currently running. This might
    - * be on the current CPU, which just calls the function directly
    + * be on the current CPU, which just calls the function directly. This will
    + * retry due to any failures in smp_call_function_single(), such as if the
    + * task_cpu() goes offline concurrently.
    *
    - * returns: @func return value, or
    - * -ESRCH - when the process isn't running
    - * -EAGAIN - when the process moved away
    + * returns @func return value or -ESRCH when the process isn't running
    */
    static int
    task_function_call(struct task_struct *p, remote_function_f func, void *info)
    @@ -110,11 +110,16 @@ task_function_call(struct task_struct *p, remote_function_f func, void *info)
    };
    int ret;

    - do {
    - ret = smp_call_function_single(task_cpu(p), remote_function, &data, 1);
    - if (!ret)
    - ret = data.ret;
    - } while (ret == -EAGAIN);
    + for (;;) {
    + ret = smp_call_function_single(task_cpu(p), remote_function,
    + &data, 1);
    + ret = !ret ? data.ret : -EAGAIN;
    +
    + if (ret != -EAGAIN)
    + break;
    +
    + cond_resched();
    + }

    return ret;
    }
    --
    2.25.1
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