Re: [PATCH] apei/ghes: Use xchg_release() for updating new cache slot instead of cmpxchg()

Ard Biesheuvel
2022-10-25T08:49:01Z

On Mon, 24 Oct 2022 at 17:43, Borislav Petkov <bp@alien8.de> wrote:
>
> Ok,
>
> here's what I've done to it, holler if something's still missing.
>
> @rjw, if you wanna take this through your tree, it should work too - it
> is unrelated to the ghes_edac changes we're doing. Or I can carry it,
> whatever you prefer.
>

Thanks for summarizing that, I'm sure it will be helpful some day :-)

> ---
> From: Ard Biesheuvel <ardb@kernel.org>
>
> Some documentation first, about how this machinery works:
>
> It seems, the intent of the GHES error records cache is to collect
> already reported errors - see the ghes_estatus_cached() checks. There's
> even a sentence trying to say what this does:
>
> /*
> * GHES error status reporting throttle, to report more kinds of
> * errors, instead of just most frequently occurred errors.
> */
>
> New elements are added to the cache this way:
>
> if (!ghes_estatus_cached(estatus)) {
> if (ghes_print_estatus(NULL, ghes->generic, estatus))
> ghes_estatus_cache_add(ghes->generic, estatus);
>
> The intent being, once this new error record is reported, it gets cached
> so that it doesn't get reported for a while due to too many, same-type
> error records getting reported in burst-like scenarios. I.e., new,
> unreported error types can have a higher chance of getting reported.
>
> Now, the loop in ghes_estatus_cache_add() is trying to pick out the
> oldest element in there. Meaning, something which got reported already
> but a long while ago, i.e., a LRU-type scheme.
>
> And the cmpxchg() is there presumably to make sure when that selected
> element slot_cache is removed, it really *is* that element that gets
> removed and not one which replaced it in the meantime.
>
> Now, ghes_estatus_cache_add() selects a slot, and either succeeds in
> replacing its contents with a pointer to a newly 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 being done here, 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.
>
> And "dropped" here means, the item doesn't get added to the cache so
> the next time it is seen, it'll get reported again and an insertion
> attempt will be done again. Eventually, it'll get inserted and all those
> times when the insertion fails, the item will get reported although the
> cache is supposed to prevent that and "ratelimit" those repeated error
> records. Not a big deal in any case.
>
> This means the cmpxchg() and the special case are not necessary.
> Therefore, just drop the existing item unconditionally.
>
> Move the xchg_release() and call_rcu() out of rcu_read_lock/unlock
> section since there is no actually dereferencing the pointer at all.
>
> [ bp:
> - Flesh out and summarize what was discussed on the thread now
> that that cache contraption is understood;
> - Touch up code style. ]
>
> 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>
> Signed-off-by: Borislav Petkov <bp@suse.de>
> Link: https://lore.kernel.org/r/20221010023559.69655-7-justin.he@arm.com
> ---
> drivers/acpi/apei/ghes.c | 60 ++++++++++++++++++++++------------------
> 1 file changed, 33 insertions(+), 27 deletions(-)
>
> diff --git a/drivers/acpi/apei/ghes.c b/drivers/acpi/apei/ghes.c
> index 249cd01cb920..6164bf737ee6 100644
> --- a/drivers/acpi/apei/ghes.c
> +++ b/drivers/acpi/apei/ghes.c
> @@ -154,7 +154,7 @@ 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;
> @@ -789,48 +789,42 @@ 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)
> +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 __rcu *victim;
> + int i, slot = -1, count;
>
> new_cache = ghes_estatus_cache_alloc(generic, estatus);
> - if (new_cache == NULL)
> + if (!new_cache)
> return;
> +
> rcu_read_lock();
> now = sched_clock();
> for (i = 0; i < GHES_ESTATUS_CACHES_SIZE; i++) {
> 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);
> @@ -839,18 +833,30 @@ 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);
> rcu_read_unlock();
> +
> + 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(&unrcu_pointer(victim)->rcu,
> + ghes_estatus_cache_rcu_free);
> + }
> }
>
> static void __ghes_panic(struct ghes *ghes,
> --
> 2.35.1
>
> --
> Regards/Gruss,
> Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette