Re: [PATCH v2 12/19] qcow2: Fix cache_clean_timer

[Kevin Wolf <kwolf@redhat.com>]

Am 10.11.2025 um 16:48 hat Hanna Czenczek geschrieben:
> The cache-cleaner runs as a timer CB in the BDS AioContext.  With
> multiqueue, it can run concurrently to I/O requests, and because it does
> not take any lock, this can break concurrent cache accesses, corrupting
> the image.  While the chances of this happening are low, it can be
> reproduced e.g. by modifying the code to schedule the timer CB every
> 5 ms (instead of at most once per second) and modifying the last (inner)
> while loop of qcow2_cache_clean_unused() like so:
> 
>     while (i < c->size && can_clean_entry(c, i)) {
>         for (int j = 0; j < 1000 && can_clean_entry(c, i); j++) {
>             usleep(100);
>         }
>         c->entries[i].offset = 0;
>         c->entries[i].lru_counter = 0;
>         i++;
>         to_clean++;
>     }
> 
> i.e. making it wait on purpose for the point in time where the cache is
> in use by something else.
> 
> The solution chosen for this in this patch is not the best solution, I
> hope, but I admittedly can’t come up with anything strictly better.
> 
> We can protect from concurrent cache accesses either by taking the
> existing s->lock, or we introduce a new (non-coroutine) mutex
> specifically for cache accesses.  I would prefer to avoid the latter so
> as not to introduce additional (very slight) overhead.
> 
> Using s->lock, which is a coroutine mutex, however means that we need to
> take it in a coroutine, so the timer must run in a coroutine.  We can
> transform it from the current timer CB style into a coroutine that
> sleeps for the set interval.  As a result, however, we can no longer
> just deschedule the timer to instantly guarantee it won’t run anymore,
> but have to await the coroutine’s exit.
> 
> (Note even before this patch there were places that may not have been so
> guaranteed after all: Anything calling cache_clean_timer_del() from the
> QEMU main AioContext could have been running concurrently to an existing
> timer CB invocation.)
> 
> Polling to await the timer to actually settle seems very complicated for
> something that’s rather a minor problem, but I can’t come up with any
> better solution that doesn’t again just overlook potential problems.
> 
> (Not Cc-ing qemu-stable, as the issue is quite unlikely to be hit, and
> I’m not too fond of this solution.)
> 
> Signed-off-by: Hanna Czenczek <hreitz@redhat.com>

> -static void cache_clean_timer_cb(void *opaque)
> +static void coroutine_fn cache_clean_timer(void *opaque)
>  {
> -    BlockDriverState *bs = opaque;
> -    BDRVQcow2State *s = bs->opaque;
> -    qcow2_cache_clean_unused(s->l2_table_cache);
> -    qcow2_cache_clean_unused(s->refcount_block_cache);
> -    timer_mod(s->cache_clean_timer, qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL) +
> -              (int64_t) s->cache_clean_interval * 1000);
> +    BDRVQcow2State *s = opaque;
> +    uint64_t wait_ns;
> +
> +    WITH_QEMU_LOCK_GUARD(&s->lock) {
> +        wait_ns = s->cache_clean_interval * NANOSECONDS_PER_SECOND;
> +    }
> +
> +    while (wait_ns > 0) {
> +        qemu_co_sleep_ns_wakeable(&s->cache_clean_timer_wake,
> +                                  QEMU_CLOCK_VIRTUAL, wait_ns);
> +
> +        WITH_QEMU_LOCK_GUARD(&s->lock) {
> +            if (s->cache_clean_interval > 0) {
> +                qcow2_cache_clean_unused(s->l2_table_cache);
> +                qcow2_cache_clean_unused(s->refcount_block_cache);
> +            }
> +
> +            wait_ns = s->cache_clean_interval * NANOSECONDS_PER_SECOND;
> +        }
> +    }
> +
> +    WITH_QEMU_LOCK_GUARD(&s->lock) {
> +        s->cache_clean_timer_co = NULL;
> +        qemu_co_queue_restart_all(&s->cache_clean_timer_exit);
> +    }
>  }

> +/**
> + * Delete the cache clean timer and await any yet running instance.
> + * Called holding s->lock.
> + */
> +static void coroutine_fn
> +cache_clean_timer_co_locked_del_and_wait(BlockDriverState *bs)
> +{
> +    BDRVQcow2State *s = bs->opaque;
> +
> +    if (s->cache_clean_timer_co) {
> +        s->cache_clean_interval = 0;
> +        qemu_co_sleep_wake(&s->cache_clean_timer_wake);
> +        qemu_co_queue_wait(&s->cache_clean_timer_exit, &s->lock);
>      }
>  }

I don't want to count the number of context switches this dance between
cache_clean_timer_co_locked_del_and_wait() and cache_clean_timer()
takes! Good that it's not a hot path. :-)

> +/* s_locked specifies whether s->lock is held or not */
>  static void qcow2_update_options_commit(BlockDriverState *bs,
> -                                        Qcow2ReopenState *r)
> +                                        Qcow2ReopenState *r,
> +                                        bool s_locked)
>  {
>      BDRVQcow2State *s = bs->opaque;
>      int i;
>  
> +    /*
> +     * We need to stop the cache-clean-timer before destroying the metadata
> +     * table caches
> +     */
> +    if (s_locked) {
> +        cache_clean_timer_co_locked_del_and_wait(bs);
> +    } else {
> +        cache_clean_timer_del_and_wait(bs);
> +    }
> +
>      if (s->l2_table_cache) {
>          qcow2_cache_destroy(s->l2_table_cache);
>      }
> @@ -1228,6 +1312,10 @@ static void qcow2_update_options_commit(BlockDriverState *bs,
>      }
>      s->l2_table_cache = r->l2_table_cache;
>      s->refcount_block_cache = r->refcount_block_cache;
> +
> +    s->cache_clean_interval = r->cache_clean_interval;
> +    cache_clean_timer_init(bs, bdrv_get_aio_context(bs));
> +
>      s->l2_slice_size = r->l2_slice_size;
>  
>      s->overlap_check = r->overlap_check;
> @@ -1239,12 +1327,6 @@ static void qcow2_update_options_commit(BlockDriverState *bs,
>  
>      s->discard_no_unref = r->discard_no_unref;
>  
> -    if (s->cache_clean_interval != r->cache_clean_interval) {
> -        cache_clean_timer_del(bs);
> -        s->cache_clean_interval = r->cache_clean_interval;
> -        cache_clean_timer_init(bs, bdrv_get_aio_context(bs));
> -    }
> -
>      qapi_free_QCryptoBlockOpenOptions(s->crypto_opts);
>      s->crypto_opts = r->crypto_opts;
>  }

Is there any specific reason why you move cache_clean_timer_init()
earlier? I don't see an actual problem with the code as it is after this
change, but s->l2_slice_size is related to the cache in a way, so it
would feel safer if the cache cleaner were only started once all the
settings are updated and not only those that it actually happens to
access at the moment.

Kevin