From: Barret Rhoden <brho@xxxxxxxxxx> commit 2ed6edd33a214bca02bd2b45e3fc3038a059436b upstream. 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@xxxxxxxxxxxxxxxxxxxxxxxxx Signed-off-by: Barret Rhoden <brho@xxxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Link: https://lkml.kernel.org/r/20200414222920.121401-1-brho@xxxxxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- kernel/events/core.c | 23 ++++++++++++++--------- 1 file changed, 14 insertions(+), 9 deletions(-) --- 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 }; 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; }
