Re: [RFC PATCH 02/11] samples/damon/mtier: do not stop first context for damon_start() failure

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] ABBA deadlock in damon_start() during error recovery when rolling back successfully started contexts.
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commit c411cd45182c74c483bd6bd5d58aea28123f720d
Author: SJ Park <sj@kernel.org>

samples/damon/mtier: do not stop first context for damon_start() failure

damon_start() was able to fail while keeping the first context running.
mtier hence stops the first context in the case. damon_start() is
refactored to avoid that. The error handling is no longer necessary.
Remove it.

> diff --git a/samples/damon/mtier.c b/samples/damon/mtier.c
> index e567f4edd80ea..90881e8bd441f 100644
> --- a/samples/damon/mtier.c
> +++ b/samples/damon/mtier.c
> @@ -193,8 +193,6 @@ static int damon_sample_mtier_start(void)
>  	if (!err)
>  		return 0;
>  
> -	if (damon_is_running(ctxs[0]))
> -		damon_stop(ctxs, 1);

[Severity: High]
Does the refactored error handling in damon_start() introduce a deadlock?

Since damon_start() is called with nr_ctxs = 2 here, if the first context
starts successfully but the second one fails, the error recovery path in
damon_start() could trigger an ABBA deadlock.

If damon_start() calls damon_stop() on the error path while holding
damon_lock:

mm/damon/core.c:damon_start() {
    ...
		err = __damon_start(ctxs[i]);
		if (err) {
			damon_stop(ctxs, i);
			break;
		}
    ...
}

damon_stop() blocks until the kdamond_fn() thread exits. However, kdamond_fn()
attempts to acquire damon_lock at its exit path:

mm/damon/core.c:kdamond_fn() {
    ...
	mutex_lock(&damon_lock);
	nr_running_ctxs--;
	if (!nr_running_ctxs && running_exclusive_ctxs)
		running_exclusive_ctxs = false;
	mutex_unlock(&damon_lock);
    ...
}

Could this cause kdamond_fn() to hang waiting for damon_lock, which is still
held by damon_start(), resulting in a deadlock?

>  	damon_destroy_ctx(ctxs[0]);
>  	damon_destroy_ctx(ctxs[1]);
>  	return err;

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Sashiko AI review · https://sashiko.dev/#/patchset/20260704181135.132956-1-sj@kernel.org?part=2