perf: Add cond_resched() to task_function_call() (2ed6edd3) · Commits · 戴 / test

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

kernel/events/core.c

+14 −9

Original line number	Diff line number	Diff line
		@@ -95,11 +95,11 @@ 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)
		@@ -112,11 +112,16 @@ 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;
		}

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