linux-kernel - Re: [PATCH 3/4] mm: replace rw_semaphore with atomic_t in vma

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Message-ID: <CAJuCfpGRdYkbJ3DkyNZPwsZqu29rnqGdOY9B+M1dL14Cu79XDg@mail.gmail.com>
Date: Mon, 11 Nov 2024 16:59:30 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org, willy@...radead.org, 
	liam.howlett@...cle.com, lorenzo.stoakes@...cle.com, mhocko@...e.com, 
	vbabka@...e.cz, hannes@...xchg.org, mjguzik@...il.com, oliver.sang@...el.com, 
	mgorman@...hsingularity.net, david@...hat.com, peterx@...hat.com, 
	oleg@...hat.com, paulmck@...nel.org, brauner@...nel.org, dhowells@...hat.com, 
	hdanton@...a.com, hughd@...gle.com, minchan@...gle.com, jannh@...gle.com, 
	shakeel.butt@...ux.dev, souravpanda@...gle.com, pasha.tatashin@...een.com, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH 3/4] mm: replace rw_semaphore with atomic_t in vma_lock

On Mon, Nov 11, 2024 at 4:35 PM Davidlohr Bueso <dave@...olabs.net> wrote:
>
> On Mon, 11 Nov 2024, Suren Baghdasaryan wrote:
>
> >@@ -787,7 +893,10 @@ static inline void vma_start_write(struct vm_area_struct *vma)
> >        * we should use WRITE_ONCE() for cleanliness and to keep KCSAN happy.
> >        */
> >       WRITE_ONCE(vma->vm_lock_seq, mm_lock_seq);
> >-      up_write(&vma->vm_lock.lock);
> >+      /* Write barrier to ensure vm_lock_seq change is visible before count */
> >+      smp_wmb();
> >+      rwsem_release(&vma->vm_lock.dep_map, _RET_IP_);
> >+      atomic_set(&vma->vm_lock.count, VMA_LOCK_UNLOCKED);
>
> Too many barriers here. Just do atomic_set_release and remove that
> smp_wmb(). And what you are doing is really ensuring nothing leaks out
> of the critical region, so that comment should also be more generic.

Uh, yes. I missed that.

>
> I would expect regression testing to catch this sort of thing.

Well, it's in vma_start_write(), which is in the write-locking path.
Maybe that's why it's not very visible.

>
> ...
>
> > #ifdef CONFIG_PER_VMA_LOCK
> >+              struct wait_queue_head vma_writer_wait;
>
> You might want to use rcuwait here instead, which is much more
> optimized for the single waiter requirement vmas have.

Thanks for the suggestion! I'll give it a try.

>
> Thanks,
> Davidlohr