Re: [PATCH v8 6/7] apei/ghes: Use unrcu_pointer for cmpxchg

From: Ard Biesheuvel <ardb@kernel.org>
To: Justin He <Justin.He@arm.com>
Cc: Borislav Petkov <bp@alien8.de>, Len Brown <lenb@kernel.org>,
	James Morse <James.Morse@arm.com>,
	Tony Luck <tony.luck@intel.com>,
	Mauro Carvalho Chehab <mchehab@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Robert Richter <rric@kernel.org>,
	Robert Moore <robert.moore@intel.com>,
	Qiuxu Zhuo <qiuxu.zhuo@intel.com>,
	Yazen Ghannam <yazen.ghannam@amd.com>,
	Jan Luebbe <jlu@pengutronix.de>,
	Khuong Dinh <khuong@os.amperecomputing.com>,
	Kani Toshi <toshi.kani@hpe.com>,
	"linux-acpi@vger.kernel.org" <linux-acpi@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-edac@vger.kernel.org" <linux-edac@vger.kernel.org>,
	"devel@acpica.org" <devel@acpica.org>,
	"Rafael J . Wysocki" <rafael@kernel.org>,
	Shuai Xue <xueshuai@linux.alibaba.com>,
	Jarkko Sakkinen <jarkko@kernel.org>,
	"linux-efi@vger.kernel.org" <linux-efi@vger.kernel.org>,
	kernel test robot <lkp@intel.com>
Subject: Re: [PATCH v8 6/7] apei/ghes: Use unrcu_pointer for cmpxchg
Date: Mon, 17 Oct 2022 11:27:17 +0200	[thread overview]
Message-ID: <CAMj1kXFNEtGYBefwcSLvs8XokyiZXDypC9mCgZhCkbVkS09Jog@mail.gmail.com> (raw)
In-Reply-To: <DBBPR08MB4538066E9C43FCF2983C8B60F7299@DBBPR08MB4538.eurprd08.prod.outlook.com>

Hi Justin,

On Mon, 17 Oct 2022 at 10:47, Justin He <Justin.He@arm.com> wrote:
>
> Hi Ard
>
> > -----Original Message-----
> > Subject: Re: [PATCH v8 6/7] apei/ghes: Use unrcu_pointer for cmpxchg
> >
> > On Fri, 14 Oct 2022 at 17:11, Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > On Fri, Oct 14, 2022 at 04:31:37PM +0200, Ard Biesheuvel wrote:
> > > > +       if (slot != -1) {
> > > > +               /*
> > > > +                * Use release semantics to ensure that
> > ghes_estatus_cached()
> > > > +                * running on another CPU will see the updated cache
> > fields if
> > > > +                * it can see the new value of the pointer.
> > > > +                */
> > > > +               victim = xchg_release(ghes_estatus_caches + slot,
> > > > +
> > RCU_INITIALIZER(new_cache));
> > > > +
> > > > +               /*
> > > > +                * At this point, victim may point to a cached item
> > different
> > > > +                * from the one based on which we selected the slot.
> > Instead of
> > > > +                * going to the loop again to pick another slot, let's
> > just
> > > > +                * drop the other item anyway: this may cause a false
> > cache
> > > > +                * miss later on, but that won't cause any problems.
> > > > +                */
> > > > +               if (victim) {
> > > > +                       call_rcu(&rcu_dereference(victim)->rcu,
> > > > +                                ghes_estatus_cache_rcu_free);
> > >                 }
> > >
> > > I think you can use unrcu_pointer() here instead, there should not be
> > > a data dependency since the ->rcu member itself should be otherwise
> > > unused (and if it were, we wouldn't care about its previous content anyway).
> > >
> > > But only Alpha cares about that distinction anyway, so *shrug*.
> > >
> >
> > Ah yeah good point - and we are not actually dereferencing the pointer at all
> > here, just adding an offset to get at the address of the rcu member.
> >
> > So we can take this block out of the rcu_read_lock() section as well.
> >
> >
> > > While I much like the xchg() variant; I still don't really fancy the
> > > verbage the sparse nonsense makes us do.
> > >
> > >                 victim = xchg_release(&ghes_estatus_caches[slot],
> > new_cache);
> > >                 if (victim)
> > >                         call_rcu(&victim->rcu,
> > > ghes_estatus_cache_rcu_free);
> > >
> > > is much nicer code.
> > >
> > > Over all; I'd simply ignore sparse (I often do).
> > >
> >
> > No disagreement there.
>
> What do you think of the updated patch:
>
> apei/ghes: Use xchg() for updating new cache slot instead of
>  cmpxchg()
>
> From: Ard Biesheuvel <ardb@kernel.org>
>
> ghes_estatus_cache_add() selects a slot, and either succeeds in
> replacing its contents with a pointer to a new cached item, or it just
> gives up and frees the new item again, without attempting to select
> another slot even if one might be available.
>
> Since only inserting new items is needed, the race can only cause a failure
> if the selected slot was updated with another new item concurrently,
> which means that it is arbitrary which of those two items gets
> dropped. This means the cmpxchg() and the special case are not necessary,
> and hence just drop the existing item unconditionally. Note that this
> does not result in loss of error events, it simply means we might
> cause a false cache miss, and report the same event one additional
> time in quick succession even if the cache should have prevented that.
>

Please add a line here

Co-developed-by: Jia He <justin.he@arm.com>

> Signed-off-by: Jia He <justin.he@arm.com>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
> [Justin]: I removed __rcu annotation of victim, removed the RCU_INITIALIZER
> cast and added the unptr for xchg.
>
> drivers/acpi/apei/ghes.c | 44 ++++++++++++++++++++--------------------
>  1 file changed, 22 insertions(+), 22 deletions(-)
>
> diff --git a/drivers/acpi/apei/ghes.c b/drivers/acpi/apei/ghes.c
> index 27c72b175e4b..5fc8a135450b 100644
> --- a/drivers/acpi/apei/ghes.c
> +++ b/drivers/acpi/apei/ghes.c
> @@ -150,7 +150,7 @@ struct ghes_vendor_record_entry {
>  static struct gen_pool *ghes_estatus_pool;
>  static unsigned long ghes_estatus_pool_size_request;
>
> -static struct ghes_estatus_cache *ghes_estatus_caches[GHES_ESTATUS_CACHES_SIZE];
> +static struct ghes_estatus_cache __rcu *ghes_estatus_caches[GHES_ESTATUS_CACHES_SIZE];
>  static atomic_t ghes_estatus_cache_alloced;
>
>  static int ghes_panic_timeout __read_mostly = 30;
> @@ -785,31 +785,26 @@ static struct ghes_estatus_cache *ghes_estatus_cache_alloc(
>         return cache;
>  }
>
> -static void ghes_estatus_cache_free(struct ghes_estatus_cache *cache)
> +static void ghes_estatus_cache_rcu_free(struct rcu_head *head)
>  {
> +       struct ghes_estatus_cache *cache;
>         u32 len;
>
> +       cache = container_of(head, struct ghes_estatus_cache, rcu);
>         len = cper_estatus_len(GHES_ESTATUS_FROM_CACHE(cache));
>         len = GHES_ESTATUS_CACHE_LEN(len);
>         gen_pool_free(ghes_estatus_pool, (unsigned long)cache, len);
>         atomic_dec(&ghes_estatus_cache_alloced);
>  }
>
> -static void ghes_estatus_cache_rcu_free(struct rcu_head *head)
> -{
> -       struct ghes_estatus_cache *cache;
> -
> -       cache = container_of(head, struct ghes_estatus_cache, rcu);
> -       ghes_estatus_cache_free(cache);
> -}
> -
>  static void ghes_estatus_cache_add(
>         struct acpi_hest_generic *generic,
>         struct acpi_hest_generic_status *estatus)
>  {
>         int i, slot = -1, count;
>         unsigned long long now, duration, period, max_period = 0;
> -       struct ghes_estatus_cache *cache, *slot_cache = NULL, *new_cache;
> +       struct ghes_estatus_cache *cache, *new_cache;
> +       struct ghes_estatus_cache *victim;
>
>         new_cache = ghes_estatus_cache_alloc(generic, estatus);
>         if (new_cache == NULL)
> @@ -820,13 +815,11 @@ static void ghes_estatus_cache_add(
>                 cache = rcu_dereference(ghes_estatus_caches[i]);
>                 if (cache == NULL) {
>                         slot = i;
> -                       slot_cache = NULL;
>                         break;
>                 }
>                 duration = now - cache->time_in;
>                 if (duration >= GHES_ESTATUS_IN_CACHE_MAX_NSEC) {
>                         slot = i;
> -                       slot_cache = cache;
>                         break;
>                 }
>                 count = atomic_read(&cache->count);
> @@ -835,17 +828,24 @@ static void ghes_estatus_cache_add(
>                 if (period > max_period) {
>                         max_period = period;
>                         slot = i;
> -                       slot_cache = cache;
>                 }
>         }
> -       /* new_cache must be put into array after its contents are written */
> -       smp_wmb();
> -       if (slot != -1 && cmpxchg(ghes_estatus_caches + slot,
> -                                 slot_cache, new_cache) == slot_cache) {
> -               if (slot_cache)
> -                       call_rcu(&slot_cache->rcu, ghes_estatus_cache_rcu_free);
> -       } else
> -               ghes_estatus_cache_free(new_cache);
> +       if (slot != -1) {
> +               /*
> +                * Use release semantics to ensure that ghes_estatus_cached()
> +                * running on another CPU will see the updated cache fields if
> +                * it can see the new value of the pointer.

Please move the comment back where it was. 'At this point' is now
ambiguous because victim has not been assigned yet.

> +                * At this point, victim may point to a cached item different
> +                * from the one based on which we selected the slot. Instead of
> +                * going to the loop again to pick another slot, let's just
> +                * drop the other item anyway: this may cause a false cache
> +                * miss later on, but that won't cause any problems.
> +                */
> +               victim = unrcu_pointer(xchg_release(&ghes_estatus_caches[slot],
> +                                       new_cache));

Doesn't this still trigger the sparse warning on x86?

> +               if (victim)
> +                       call_rcu(&victim->rcu, ghes_estatus_cache_rcu_free);

I think it is better to add back the __rcu annotation to 'victim', and
change this line to

call_rcu(&unrcu_pointer(victim)->rcu, ghes_estatus_cache_rcu_free);

> +       }
>         rcu_read_unlock();

This can now be moved before the if()

>  }
>
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