Re: [RFC PATCH v1] mm: improve call_controls_lock

[SeongJae Park <sj@kernel.org>]

On Wed, 31 Dec 2025 11:15:00 +0900 JaeJoon Jung <rgbi3307@gmail.com> wrote:

> On Tue, 30 Dec 2025 at 00:23, SeongJae Park <sj@kernel.org> wrote:
> >
> > Hello Asier,
> >
> >
> > Thank you for sending this patch!
> >
> > On Mon, 29 Dec 2025 14:55:32 +0000 Asier Gutierrez <gutierrez.asier@huawei-partners.com> wrote:
> >
> > > This is a minor patch set for a call_controls_lock synchronization improvement.
> >
> > Please break description lines to not exceed 75 characters per line.
> >
> > >
> > > Spinlocks are faster than mutexes, even when the mutex takes the fast
> > > path. Hence, this patch replaces the mutex call_controls_lock with a spinlock.
> >
> > But call_controls_lock is not being used on performance critical part.
> > Actually, most of DAMON code is not performance critical.  I really appreciate
> > your patch, but I have to say I don't think this change is really needed now.
> > Please let me know if I'm missing something.
> 
> Paradoxically, when it comes to locking, spin_lock is better than
> mutex_lock
> because "most of DAMON code is not performance critical."
> 
> DAMON code only accesses the ctx belonging to kdamond itself. For
> example:
> kdamond.0 --> ctx.0
> kdamond.1 --> ctx.1
> kdamond.2 --> ctx.2
> kdamond.# --> ctx.#
> 
> There is no cross-approach as shown below:
> kdamond.0 --> ctx.1
> kdamond.1 --> ctx.2
> kdamond.2 --> ctx.0
> 
> Only the data belonging to kdamond needs to be resolved for concurrent access.
> most DAMON code needs to lock/unlock briefly when add/del linked
> lists,
> so spin_lock is effective.

I don't disagree this.  Both spinlock and mutex effectively work for DAMON's
locking usages.

> If you handle it with a mutex, it becomes
> more
> complicated because the rescheduling occurs as a context switch occurs
> inside the kernel.

Can you please elaborate what kind of complexities you are saying about?
Adding some examples would be nice.

> Moreover, since the call_controls_lock that is
> currently
> being raised as a problem only occurs in two places, the kdamon_call()
> loop
> and the damon_call() function, it is effective to handle it with a
> spin_lock
> as shown below.
> 
> @@ -1502,14 +1501,15 @@ int damon_call(struct damon_ctx *ctx, struct
> damon_call_control *control)
>         control->canceled = false;
>         INIT_LIST_HEAD(&control->list);
> 
> -       mutex_lock(&ctx->call_controls_lock);
> +       spin_lock(&ctx->call_controls_lock);
> +       /* damon_is_running */
>         if (ctx->kdamond) {
>                 list_add_tail(&control->list, &ctx->call_controls);
>         } else {
> -               mutex_unlock(&ctx->call_controls_lock);
> +               spin_unlock(&ctx->call_controls_lock);
>                 return -EINVAL;
>         }
> -       mutex_unlock(&ctx->call_controls_lock);
> +       spin_unlock(&ctx->call_controls_lock);
> 
>         if (control->repeat)
>                 return 0;

Are you saying the above diff can fix the damon_call() use-after-free bug [1]?
Can you please elaborate why you think so?

[1] https://lore.kernel.org/20251231012315.75835-1-sj@kernel.org


Thanks,
SJ

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