On Thu, 27 Aug 2026, Ming Hung Tsai wrote:
> On Wed, Aug 26, 2026 at 3:22 AM Benjamin Marzinski <[email protected]> > wrote: > > > > dm cache used a rw_semaphore for background_work_lock. Write locks on > > rw_semaphores have strict owner semantics, but there was no guarantee > > that the process that locked background_work_lock was the same process > > that unlocked it. This can be easily seen using a kernel compiled with > > CONFIG_DEBUG_RWSEMS. Given a dm cache device <cache>, run: 'dmsetup > > suspend <cache> && dmsetup resume <cache>'. This will tigger a kernel > > warning: > > > > DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) && > > !rwsem_test_oflags(sem, RWSEM_NONSPINNABLE)) > > > > triggered by cache_resume(). To fix this, switch from a rw_semaphore to > > a spinlock and a wait queue. dm cache already hasd a wait queue and > > associated counter, migration_wait and nr_allocated_migrations, that was > > getting woken up when background work was getting completed, but wasn't > > actually used by anything. This is replaced by the background_work queue > > and counter. > > > > Fixes: b29d4986d0da ("dm cache: significant rework to leverage > > dm-bio-prison-v2") > > Signed-off-by: Benjamin Marzinski <[email protected]> > > --- > > Thanks Ben. I confirmed the patch fixed it. > > Just one minor nit: checkpatch prefers 'unsigned int' for background_work_nr. Hi Personally, I prefer 'unsigned'. There is no reason why to type more tokens if we don't have to. Mikulas > Worth noting before the commit b29d4986d0da ("dm cache: significant > rework to leverage dm-bio-prison-v2"), dm cache waited for migrations > via wait_for_migrations(), which did a wait_event() on migration_wait > and nr_allocated_migrations. That commit replaced it with > rw_semaphores and used lockdep_off() to suppress warnings, before > DEBUG_RWSEMS existed. > > Reviewed-by: Ming-Hung Tsai <[email protected]> > > > drivers/md/dm-cache-target.c | 55 ++++++++++++++++++------------------ > > 1 file changed, 28 insertions(+), 27 deletions(-) > > > > diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c > > index 33dbc71b730f..d8d9c63d2a67 100644 > > --- a/drivers/md/dm-cache-target.c > > +++ b/drivers/md/dm-cache-target.c > > @@ -340,8 +340,6 @@ struct cache { > > struct list_head invalidation_requests; > > > > sector_t migration_threshold; > > - wait_queue_head_t migration_wait; > > - atomic_t nr_allocated_migrations; > > > > /* > > * The number of in flight migrations that are performing > > @@ -397,7 +395,11 @@ struct cache { > > bool loaded_mappings:1; > > bool loaded_discards:1; > > > > - struct rw_semaphore background_work_lock; > > + /* background work management */ > > + bool background_work_allowed; > > + unsigned background_work_nr; > > + spinlock_t background_work_lock; > > + wait_queue_head_t background_work_wait; > > > > struct batcher committer; > > struct work_struct commit_ws; > > @@ -488,19 +490,13 @@ static struct dm_cache_migration > > *alloc_migration(struct cache *cache) > > memset(mg, 0, sizeof(*mg)); > > > > mg->cache = cache; > > - atomic_inc(&cache->nr_allocated_migrations); > > > > return mg; > > } > > > > static void free_migration(struct dm_cache_migration *mg) > > { > > - struct cache *cache = mg->cache; > > - > > - if (atomic_dec_and_test(&cache->nr_allocated_migrations)) > > - wake_up(&cache->migration_wait); > > - > > - mempool_free(mg, &cache->migration_pool); > > + mempool_free(mg, &mg->cache->migration_pool); > > } > > > > /*----------------------------------------------------------------*/ > > @@ -1030,34 +1026,39 @@ static void calc_discard_block_range(struct cache > > *cache, struct bio *bio, > > > > static void prevent_background_work(struct cache *cache) > > { > > - lockdep_off(); > > - down_write(&cache->background_work_lock); > > - lockdep_on(); > > + spin_lock_irq(&cache->background_work_lock); > > + cache->background_work_allowed = false; > > + wait_event_lock_irq(cache->background_work_wait, > > + cache->background_work_nr == 0, > > + cache->background_work_lock); > > + spin_unlock_irq(&cache->background_work_lock); > > } > > > > static void allow_background_work(struct cache *cache) > > { > > - lockdep_off(); > > - up_write(&cache->background_work_lock); > > - lockdep_on(); > > + spin_lock_irq(&cache->background_work_lock); > > + cache->background_work_allowed = true; > > + spin_unlock_irq(&cache->background_work_lock); > > } > > > > static bool background_work_begin(struct cache *cache) > > { > > bool r; > > > > - lockdep_off(); > > - r = down_read_trylock(&cache->background_work_lock); > > - lockdep_on(); > > - > > + spin_lock_irq(&cache->background_work_lock); > > + r = cache->background_work_allowed; > > + if (r) > > + cache->background_work_nr++; > > + spin_unlock_irq(&cache->background_work_lock); > > return r; > > } > > > > static void background_work_end(struct cache *cache) > > { > > - lockdep_off(); > > - up_read(&cache->background_work_lock); > > - lockdep_on(); > > + spin_lock_irq(&cache->background_work_lock); > > + if (--cache->background_work_nr == 0) > > + wake_up(&cache->background_work_wait); > > + spin_unlock_irq(&cache->background_work_lock); > > } > > > > /*----------------------------------------------------------------*/ > > @@ -2507,9 +2508,7 @@ static int cache_create(struct cache_args *ca, struct > > cache **result) > > > > spin_lock_init(&cache->lock); > > bio_list_init(&cache->deferred_bios); > > - atomic_set(&cache->nr_allocated_migrations, 0); > > atomic_set(&cache->nr_io_migrations, 0); > > - init_waitqueue_head(&cache->migration_wait); > > > > r = -ENOMEM; > > atomic_set(&cache->nr_dirty, 0); > > @@ -2592,8 +2591,10 @@ static int cache_create(struct cache_args *ca, > > struct cache **result) > > issue_op, cache, cache->wq); > > dm_iot_init(&cache->tracker); > > > > - init_rwsem(&cache->background_work_lock); > > - prevent_background_work(cache); > > + init_waitqueue_head(&cache->background_work_wait); > > + spin_lock_init(&cache->background_work_lock); > > + cache->background_work_allowed = false; > > + cache->background_work_nr = 0; > > > > *result = cache; > > return 0; > > -- > > 2.53.0 > > >
