Re: [RFC PATCH v1] mm: improve call_controls_lock

SeongJae Park
2026-01-01T01:51:26Z

On Thu, 1 Jan 2026 10:07:13 +0900 JaeJoon Jung <rgbi3307@gmail.com> wrote:

> On Wed, 31 Dec 2025 at 13:59, SeongJae Park <sj@kernel.org> wrote:
> >
> > 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.
>
> You probably know better than I do. What I'm saying is too general.
> spin_lock is good for short collision waits, while mutex is more efficient
> for longer ones. However, I'm saying that mutexes place a burden on the
> kernel because they schedule internally.

Thank you for added explanation. But, in your previous reply, you mentioned
"it becomes more complicated because the rescheduling occurs as a context
switch occurs inside the kernel". Your above explanation ("mutexex place a
burden on the kernel because they schedule internally") is not adding more
explanation but just a repeat, to my view. I'm asking what complexity (or,
burden) that you concern here, the internal scheduling is causing.

So, may I ask your explanation again?

>
> >
> > > 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

You didn't answer the above question.

Thanks,
SJ …