linux-kernel - Re: [PATCH v4 12/22] lib/stackdepot: use read/write lock

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Message-ID: <CANpmjNNi=JqTsfZAWDg-e4ee2v3rXmHCg7UL7ZvN92yr2Y2vUg@mail.gmail.com>
Date: Sat, 13 Jan 2024 10:30:22 +0100
From: Marco Elver <elver@...gle.com>
To: Andi Kleen <ak@...ux.intel.com>
Cc: Andrey Konovalov <andreyknvl@...il.com>, Oscar Salvador <osalvador@...e.de>, andrey.konovalov@...ux.dev, 
	Andrew Morton <akpm@...ux-foundation.org>, Alexander Potapenko <glider@...gle.com>, 
	Dmitry Vyukov <dvyukov@...gle.com>, Vlastimil Babka <vbabka@...e.cz>, kasan-dev@...glegroups.com, 
	Evgenii Stepanov <eugenis@...gle.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH v4 12/22] lib/stackdepot: use read/write lock

On Sat, 13 Jan 2024 at 10:23, Marco Elver <elver@...gle.com> wrote:
>
> On Sat, 13 Jan 2024 at 10:19, Andi Kleen <ak@...ux.intel.com> wrote:
> >
> > On Sat, Jan 13, 2024 at 10:12:21AM +0100, Marco Elver wrote:
> > > On Sat, 13 Jan 2024 at 02:24, Andi Kleen <ak@...ux.intel.com> wrote:
> > > >
> > > > On Fri, Jan 12, 2024 at 11:15:05PM +0100, Marco Elver wrote:
> > > > > +             /*
> > > > > +              * Stack traces of size 0 are never saved, and we can simply use
> > > > > +              * the size field as an indicator if this is a new unused stack
> > > > > +              * record in the freelist.
> > > > > +              */
> > > > > +             stack->size = 0;
> > > >
> > > > I would use WRITE_ONCE here too, at least for TSan.
> > >
> > > This is written with the pool_lock held.
> >
> > ...which doesn't help because the readers don't take it?
>
> This function is only refilling the freelist. Readers don't see it yet
> because it's in none of the hash table buckets. The freelist is only
> ever accessed under the lock.
>
> Once an entry is allocated from the freelist, its size is overwritten
> with something non-zero (since it then contains a stack trace). Those
> updates are released into the right hash table bucket with
> list_add_rcu() (which implies a release).
>
> Am I missing something else?

FWIW, the current version (draft) of this can be found here:
https://git.kernel.org/pub/scm/linux/kernel/git/melver/linux.git/log/?h=kasan/dev
I'll send the 2 patches next week - they should apply cleanly on
current mainline.

Thanks,
-- Marco