Re: [PATCH v2 1/1] bcache: track active bypass writes to prevent stale cache reads

["Coly Li" <colyli@fygo.io>]

Hi Ankit,

This is not an easy and simple fix, finally I find a long enough and
peaceful time to reivew the whole patch.

Thanks for your patience. I add my comments inline in your code. 

On Wed, Jun 17, 2026 at 10:33:56AM +0800, Ankit Kapoor wrote:
> A race condition exists between a read cache miss and a bypass write
> due to either congestion or sequential bypass, that causes stale data
> to be cached when the read cache miss runs concurrently with a bypass
> write targeting the same sectors. If the read cache miss fetches data
> from the backing device before the write to the backing device, stale
> data populates the cache.
> 
> The root cause is that bcache currently executes btree key
> invalidation in parallel with (or prior to) writing the actual data
> payload to the backing device. Under this sequence, a concurrent read
> path can register a cache miss and insert a placeholder key. If the
> write's btree key invalidation completes before the read finishes
> fetching old data from the backing device, the read's subsequent key
> replacement will not detect a collision, allowing stale data to
> persist in the cache.
> 
> Fix this by tracking active bypass writes. We divide the backing
> device space into 32MB chunks and track concurrent bypass writes
> using refcounts. The tracking counters are stored in dynamically
> allocated pages, minimizing memory overhead (a single 4KB page
> supports 64GB of disk space). Synchronization is handled via
> page-level spinlocks rather than a single global lock, ensuring
> minimal contention across different regions of the disk.
> 
> On a cache miss read, bcache checks if there are any active bypass
> writes overlapping the target sectors. If an active bypass write is
> detected, the read is forced to bypass the cache to ensure data
> consistency.
> 
> If the system fails to allocate a tracking page due to memory
> pressure, the bypass write proceeds untracked. To provide
> observability into this fallback, we print a rate-limited dmesg
> warning and track the allocation failures using a new
> `bypass_tracking_alloc_fails` sysfs counter.
> 
> Additionally, add a `cache_read_bypass_races` sysfs counter and a
> corresponding tracepoint to monitor these occurrences.
> 
> Signed-off-by: Ankit Kapoor <ankitkap@google.com>
> Suggested-by: Coly Li <colyli@fygo.io>
> ---
> 
> v2:
> - Addressed feedback from Coly Li regarding SSD power failures.
> - Implemented bypass write monitoring to force concurrent reads 
>   to the bypass path.
> - Referenced md RAID bitmap implementation for the architectural
>   approach as per Coly Li's suggestion


> 
>  Documentation/admin-guide/bcache.rst |   8 ++
>  drivers/md/bcache/bcache.h           |  35 +++++++
>  drivers/md/bcache/request.c          | 132 +++++++++++++++++++++++++++
>  drivers/md/bcache/stats.c            |  14 +++
>  drivers/md/bcache/stats.h            |   4 +
>  drivers/md/bcache/super.c            |  30 ++++++
>  drivers/md/bcache/sysfs.c            |   5 +
>  include/trace/events/bcache.h        |   5 +
>  8 files changed, 233 insertions(+)
> 
> diff --git a/Documentation/admin-guide/bcache.rst b/Documentation/admin-guide/bcache.rst
> index 325816edbdab..e08cfa6b0ea0 100644
> --- a/Documentation/admin-guide/bcache.rst
> +++ b/Documentation/admin-guide/bcache.rst
> @@ -507,6 +507,10 @@ cache_miss_collisions
>    cache miss, but raced with a write and data was already present (usually 0
>    since the synchronization for cache misses was rewritten)
>  
> +cache_read_bypass_races
> +  Counts instances where a cache miss read raced with a concurrent bypass
> +  write, forcing the read to bypass the cache to prevent reading stale data.
> +

I don't review this at this moment.

But I suggest to device this patch into two parts. One is the code which
solves the stale data race condition, another one is about the sysfs,
stats, statistics. Then we can focus on the core part firstly.

>  Sysfs - cache set
>  ~~~~~~~~~~~~~~~~~
>  
> @@ -592,6 +596,10 @@ bset_tree_stats
>  btree_cache_max_chain
>    Longest chain in the btree node cache's hash table
>  
> +bypass_tracking_alloc_fails
> +  Counts instances where memory allocation for bypass write tracking
> +  failed. When this occurs, a bypass write proceeds untracked.
> +
>  cache_read_races
>    Counts instances where while data was being read from the cache, the bucket
>    was reused and invalidated - i.e. where the pointer was stale after the read
> diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
> index ec9ff9715081..8e08503a698b 100644
> --- a/drivers/md/bcache/bcache.h
> +++ b/drivers/md/bcache/bcache.h
> @@ -299,6 +299,12 @@ enum stop_on_failure {
>  	BCH_CACHED_DEV_STOP_MODE_MAX,
>  };
>  
> +struct bch_bypass_page {
> +	u16		*counts;
> +	unsigned int	active;
> +	spinlock_t	lock;
> +};

The refcount type can be u32, for a 100TB backend if all the counts pages
are fully occupied and allocated, it is more or less around 100MB memory.
For such a large storage device in worst case, such memory consuming is
acceptible IMHO.

> +
>  struct cached_dev {
>  	struct list_head	list;
>  	struct bcache_device	disk;
> @@ -407,8 +413,36 @@ struct cached_dev {
>  	 */
>  #define BCH_WBRATE_UPDATE_MAX_SKIPS	15
>  	unsigned int		rate_update_retry;
> +
> +	/* For tracking active bypass writes */
> +#define BCH_BYPASS_CHUNK_SHIFT		16  /* 2^16 sectors = 32MB */
> +#define BCH_BYPASS_PAGE_COUNTERS	(PAGE_SIZE / sizeof(u16))
> +#define BCH_BYPASS_PAGE_SHIFT		(PAGE_SHIFT - 1)
> +#define BCH_BYPASS_PAGE_MASK		((1UL << BCH_BYPASS_PAGE_SHIFT) - 1)
> +	struct bch_bypass_page	*bypass_pages;
> +	unsigned long		bypass_num_pages;
>  };
>  
> +static inline unsigned long sector_to_bypass_chunk(sector_t sector)
> +{
> +	return sector >> BCH_BYPASS_CHUNK_SHIFT;
> +}
> +
> +static inline unsigned long bypass_chunk_to_page(unsigned long chunk)
> +{
> +	return chunk >> BCH_BYPASS_PAGE_SHIFT;
> +}
> +
> +static inline unsigned long bypass_chunk_to_offset(unsigned long chunk)
> +{
> +	return chunk & BCH_BYPASS_PAGE_MASK;
> +}
> +
> +static inline sector_t bypass_chunk_to_sector(unsigned long chunk)
> +{
> +	return (sector_t)chunk << BCH_BYPASS_CHUNK_SHIFT;
> +}
> +
>  enum alloc_reserve {
>  	RESERVE_BTREE,
>  	RESERVE_PRIO,
> @@ -714,6 +748,7 @@ struct cache_set {
>  	struct time_stats	btree_read_time;
>  
>  	atomic_long_t		cache_read_races;
> +	atomic_long_t		bypass_tracking_alloc_fails;
>  	atomic_long_t		writeback_keys_done;
>  	atomic_long_t		writeback_keys_failed;
>  
> diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
> index 3fa3b13a410f..426665c07394 100644
> --- a/drivers/md/bcache/request.c
> +++ b/drivers/md/bcache/request.c
> @@ -830,6 +830,126 @@ static CLOSURE_CALLBACK(cached_dev_cache_miss_done)
>  	closure_put(&d->cl);
>  }
>  
> +static void bch_bypass_write_start(struct cached_dev *dc, sector_t sector, unsigned int sectors)
> +{
> +	unsigned long start_chunk = sector_to_bypass_chunk(sector);
> +	unsigned long end_chunk = sector_to_bypass_chunk(sector + sectors - 1);
> +	unsigned long end_pg_idx = bypass_chunk_to_page(end_chunk);
> +	unsigned long chunk;
> +
> +	if (!dc->bypass_pages)
> +		return;

This is possible the bch_bypass_write_start() gets called before
dc->bypass_page is allocated? If not, I guess the above check can be
avoided.

> +
> +	if (WARN_ON_ONCE(end_pg_idx >= dc->bypass_num_pages))
> +		return;
> +
> +	for (chunk = start_chunk; chunk <= end_chunk; chunk++) {
> +		unsigned long pg_idx = bypass_chunk_to_page(chunk);
> +		unsigned long pg_off = bypass_chunk_to_offset(chunk);
> +		struct bch_bypass_page *pg = &dc->bypass_pages[pg_idx];
> +		u16 *new_counts;
> +		u16 *dup_counts = NULL;
> +
> +		spin_lock_irq(&pg->lock);
> +		if (!pg->counts) {
> +			spin_unlock_irq(&pg->lock);
> +			new_counts = kzalloc(PAGE_SIZE, __GFP_NOWARN | GFP_NOIO);

I will suggest to allocate the counts page from a mempool with GFP_NOIO,
than the code can be simpler.


> +			spin_lock_irq(&pg->lock);
> +			if (!new_counts) {
> +				if (!pg->counts) {
> +					spin_unlock_irq(&pg->lock);
> +					pr_warn_ratelimited("failed to allocate bypass write tracking page, bypass write untracked for sectors %llu-%llu\n",
> +							    (uint64_t)bypass_chunk_to_sector(chunk),
> +							    (uint64_t)bypass_chunk_to_sector(
> +								    chunk + 1) - 1);
> +					if (dc->disk.c)
> +						atomic_long_inc(
> +							&dc->disk.c->bypass_tracking_alloc_fails);
> +					continue;
> +				}
> +			}
> +			if (new_counts) {
> +				if (pg->counts)
> +					dup_counts = new_counts;
> +				else
> +					pg->counts = new_counts;
> +			}
> +		}
> +		pg->counts[pg_off]++;
> +		pg->active++;
> +		spin_unlock_irq(&pg->lock);
> +

For the above counter update, I will suggest to use RCU stuffs.


> +		kfree(dup_counts);
> +	}
> +}
> +
> +static void bch_bypass_write_end(struct cached_dev *dc, sector_t sector, unsigned int sectors)
> +{
> +	unsigned long start_chunk = sector_to_bypass_chunk(sector);
> +	unsigned long end_chunk = sector_to_bypass_chunk(sector + sectors - 1);
> +	unsigned long end_pg_idx = bypass_chunk_to_page(end_chunk);
> +	unsigned long chunk;
> +
> +	if (!dc->bypass_pages)
> +		return;
> +

If the related routines won't be called before dc->bypass_pages gets
allocated, all such above checks can be avoided.


> +	if (WARN_ON_ONCE(end_pg_idx >= dc->bypass_num_pages))
> +		return;
> +
> +	for (chunk = start_chunk; chunk <= end_chunk; chunk++) {
> +		unsigned long pg_idx = bypass_chunk_to_page(chunk);
> +		unsigned long pg_off = bypass_chunk_to_offset(chunk);
> +		struct bch_bypass_page *pg = &dc->bypass_pages[pg_idx];
> +		u16 *counts = NULL;
> +		unsigned long flags;
> +
> +		spin_lock_irqsave(&pg->lock, flags);
> +		if (pg->counts && pg->counts[pg_off] > 0) {
At this moment, you need to check whether !pg->counts or
!pg->counts[pg_off], if such condition happens, it means the code has
something fishy and should be noticed.

> +			pg->counts[pg_off]--;
> +			pg->active--;
> +			if (!pg->active) {
> +				counts = pg->counts;
> +				pg->counts = NULL;
> +			}
> +		}
> +
> +		kfree(counts);

Normally for full random workload, each counts page only has one bypass I/O,
free the page immediately after the refcount reaches 0 might not be an ideal
method. But you don't suggest you to improve this at this moment, after
current fix is solid and merged, if you have time and interest, maybe you can
continue to think about how to improve this.

> +	}
> +}
> +
> +static bool bch_has_active_bypass_writes(struct cached_dev *dc, sector_t sector,
> +					 unsigned int sectors)
> +{
> +	unsigned long start_chunk = sector_to_bypass_chunk(sector);
> +	unsigned long end_chunk = sector_to_bypass_chunk(sector + sectors - 1);
> +	unsigned long end_pg_idx = bypass_chunk_to_page(end_chunk);
> +	unsigned long chunk;
> +	bool has_active = false;
> +
> +	if (!dc->bypass_pages)
> +		return false;
> +
> +	if (WARN_ON_ONCE(end_pg_idx >= dc->bypass_num_pages))
> +		return false;
> +
> +	for (chunk = start_chunk; chunk <= end_chunk; chunk++) {
> +		unsigned long pg_idx = bypass_chunk_to_page(chunk);
> +		unsigned long pg_off = bypass_chunk_to_offset(chunk);
> +		struct bch_bypass_page *pg = &dc->bypass_pages[pg_idx];
> +
> +		spin_lock_irq(&pg->lock);
> +		if (pg->counts && pg->counts[pg_off] > 0) {
> +			has_active = true;
> +			spin_unlock_irq(&pg->lock);
> +			break;
> +		}
> +		spin_unlock_irq(&pg->lock);

The spin_lock here is uncomfortable. I feel this is one reason of the
performance killer. I suggest to look for a chance whether RCU can be
used here as a read path.

> +		spin_unlock_irqrestore(&pg->lock, flags);
> +	}
> +
> +	return has_active;
> +}
> +
>  static CLOSURE_CALLBACK(cached_dev_read_done)
>  {
>  	closure_type(s, struct search, cl);
> @@ -899,6 +1019,13 @@ static int cached_dev_cache_miss(struct btree *b, struct search *s,
>  
>  	s->cache_missed = 1;
>  
> +	if (bch_has_active_bypass_writes(dc, bio->bi_iter.bi_sector,
> +					 min(sectors, bio_sectors(bio)))) {
> +		s->iop.bypass = true;
> +		trace_bcache_read_bypass_race(bio);
> +		bch_mark_cache_read_bypass_race(s->iop.c, s->d);
> +	}
> +

This is good. But considering you mentioned the cache device util decreased,
I feel it might be too early to call bch_has_active_bypass_writes() here.

If bch_has_active_bypass_writes() can be called a bit latter, maybe the
time window for cache-miss-read to overlap bypass write can shrink and
more cache-miss-read can be cached on NVMe SSD without the stale data race
risk.

I suggest that bch_has_active_bypass_writes() check might be moved in
cached_dev_read_done() like this,

 866         if (s->iop.bio &&
 867             !test_bit(CACHE_SET_STOPPING, &s->iop.c->flags) &&
 +		 !bch_has_active_bypass_writes()) {
 868                 BUG_ON(!s->iop.replace);
 869                 closure_call(&s->iop.cl, bch_data_insert, NULL, cl);
 870         }

This location is after the placeholder key inserted into btree, and the cache-
missed data is read in memory (no matter old or new version), but before to
insert the missed data onto cache bucket and the clean key back into btree.

If this read-in data is overlapped with on flying bypass write, don't insert
the read-in data and the cleaned bkey. But the previous insertd placeholder
key will be left in btree node.

Only do this modification is not enough, new race condition will be introduced
in. So I recall your v1 patch, which serializes the invlidation key after
bypass write completed. If the bkey invalidation of the bypass write range is
explicitly serialized after the bypass write completion, such new race risks
can be avoided.

So I'd like to suggest to shrink the overlap checking time windows by moving
bch_has_active_bypass_writes() into cached_dev_read_done(), and to still send
the bkey invalidation after the bypass write completion.

I don't test it, just try whether such change may help a bit to increase
NVMe SSD util% as expected.


>  	if (s->cache_miss || s->iop.bypass) {
>  		miss = bio_next_split(bio, sectors, GFP_NOIO, &s->d->bio_split);
>  		ret = miss == bio ? MAP_DONE : MAP_CONTINUE;
> @@ -974,6 +1101,9 @@ static CLOSURE_CALLBACK(cached_dev_write_complete)
>  	closure_type(s, struct search, cl);
>  	struct cached_dev *dc = container_of(s->d, struct cached_dev, disk);
>  
> +	if (s->iop.bypass)
> +		bch_bypass_write_end(dc, s->iop.bio->bi_iter.bi_sector, bio_sectors(s->iop.bio));
> +
>  	up_read_non_owner(&dc->writeback_lock);
>  	cached_dev_bio_complete(&cl->work);
>  }
> @@ -1018,6 +1148,8 @@ static void cached_dev_write(struct cached_dev *dc, struct search *s)
>  		s->iop.bio = s->orig_bio;
>  		bio_get(s->iop.bio);
>  
> +		bch_bypass_write_start(dc, bio->bi_iter.bi_sector, bio_sectors(bio));
> +
>  		if (bio_op(bio) == REQ_OP_DISCARD &&
>  		    !bdev_max_discard_sectors(dc->bdev))
>  			goto insert_data;
> diff --git a/drivers/md/bcache/stats.c b/drivers/md/bcache/stats.c
> index 0056106495a7..df21452e106a 100644
> --- a/drivers/md/bcache/stats.c
> +++ b/drivers/md/bcache/stats.c
> @@ -47,6 +47,7 @@ read_attribute(cache_bypass_hits);
>  read_attribute(cache_bypass_misses);
>  read_attribute(cache_hit_ratio);
>  read_attribute(cache_miss_collisions);
> +read_attribute(cache_read_bypass_races);

Naming is a challenge to me. IMHO, this is a race between cache-miss read
and bypass write. The above sysfs file name is not ideal, but I don't know
how to name it better....

>  read_attribute(bypassed);
>  
>  SHOW(bch_stats)
> @@ -64,6 +65,7 @@ SHOW(bch_stats)
>  			     var(cache_hits) + var(cache_misses)));
>  
>  	var_print(cache_miss_collisions);
> +	var_print(cache_read_bypass_races);
>  	sysfs_hprint(bypassed,	var(sectors_bypassed) << 9);
>  #undef var
>  	return 0;
> @@ -85,6 +87,7 @@ static struct attribute *bch_stats_attrs[] = {
>  	&sysfs_cache_bypass_misses,
>  	&sysfs_cache_hit_ratio,
>  	&sysfs_cache_miss_collisions,
> +	&sysfs_cache_read_bypass_races,
>  	&sysfs_bypassed,
>  	NULL
>  };
> @@ -112,6 +115,7 @@ void bch_cache_accounting_clear(struct cache_accounting *acc)
>  	acc->total.cache_bypass_hits = 0;
>  	acc->total.cache_bypass_misses = 0;
>  	acc->total.cache_miss_collisions = 0;
> +	acc->total.cache_read_bypass_races = 0;
>  	acc->total.sectors_bypassed = 0;
>  }
>  
> @@ -143,6 +147,7 @@ static void scale_stats(struct cache_stats *stats, unsigned long rescale_at)
>  		scale_stat(&stats->cache_bypass_hits);
>  		scale_stat(&stats->cache_bypass_misses);
>  		scale_stat(&stats->cache_miss_collisions);
> +		scale_stat(&stats->cache_read_bypass_races);
>  		scale_stat(&stats->sectors_bypassed);
>  	}
>  }
> @@ -165,6 +170,7 @@ static void scale_accounting(struct timer_list *t)
>  	move_stat(cache_bypass_hits);
>  	move_stat(cache_bypass_misses);
>  	move_stat(cache_miss_collisions);
> +	move_stat(cache_read_bypass_races);
>  	move_stat(sectors_bypassed);
>  
>  	scale_stats(&acc->total, 0);
> @@ -212,6 +218,14 @@ void bch_mark_cache_miss_collision(struct cache_set *c, struct bcache_device *d)
>  	atomic_inc(&c->accounting.collector.cache_miss_collisions);
>  }
>  
> +void bch_mark_cache_read_bypass_race(struct cache_set *c, struct bcache_device *d)
> +{
> +	struct cached_dev *dc = container_of(d, struct cached_dev, disk);
> +
> +	atomic_inc(&dc->accounting.collector.cache_read_bypass_races);
> +	atomic_inc(&c->accounting.collector.cache_read_bypass_races);
> +}
> +
>  void bch_mark_sectors_bypassed(struct cache_set *c, struct cached_dev *dc,
>  			       int sectors)
>  {
> diff --git a/drivers/md/bcache/stats.h b/drivers/md/bcache/stats.h
> index 21b445f8af15..97d25e0d177c 100644
> --- a/drivers/md/bcache/stats.h
> +++ b/drivers/md/bcache/stats.h
> @@ -8,6 +8,7 @@ struct cache_stat_collector {
>  	atomic_t cache_bypass_hits;
>  	atomic_t cache_bypass_misses;
>  	atomic_t cache_miss_collisions;
> +	atomic_t cache_read_bypass_races;
>  	atomic_t sectors_bypassed;
>  };
>  
> @@ -19,6 +20,7 @@ struct cache_stats {
>  	unsigned long cache_bypass_hits;
>  	unsigned long cache_bypass_misses;
>  	unsigned long cache_miss_collisions;
> +	unsigned long cache_read_bypass_races;
>  	unsigned long sectors_bypassed;
>  
>  	unsigned int		rescale;
> @@ -55,6 +57,8 @@ void bch_mark_cache_accounting(struct cache_set *c, struct bcache_device *d,
>  			       bool hit, bool bypass);
>  void bch_mark_cache_miss_collision(struct cache_set *c,
>  				   struct bcache_device *d);
> +void bch_mark_cache_read_bypass_race(struct cache_set *c,
> +				     struct bcache_device *d);
>  void bch_mark_sectors_bypassed(struct cache_set *c,
>  			       struct cached_dev *dc,
>  			       int sectors);
> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
> index 97d9adb0bf96..dc584da349d9 100644
> --- a/drivers/md/bcache/super.c
> +++ b/drivers/md/bcache/super.c
> @@ -1346,6 +1346,13 @@ void bch_cached_dev_release(struct kobject *kobj)
>  {
>  	struct cached_dev *dc = container_of(kobj, struct cached_dev,
>  					     disk.kobj);
> +	if (dc->bypass_pages) {
> +		unsigned long i;
> +
> +		for (i = 0; i < dc->bypass_num_pages; i++)
> +			kfree(dc->bypass_pages[i].counts);
> +		kfree(dc->bypass_pages);
> +	}
>  	kfree(dc);
>  	module_put(THIS_MODULE);
>  }
> @@ -1407,6 +1414,25 @@ static CLOSURE_CALLBACK(cached_dev_flush)
>  	continue_at(cl, cached_dev_free, system_percpu_wq);
>  }
>  
> +static int bch_cached_dev_bypass_init(struct cached_dev *dc, sector_t sectors)
> +{
> +	unsigned long chunks = (sectors + (1UL << BCH_BYPASS_CHUNK_SHIFT) - 1) >>
> +			       BCH_BYPASS_CHUNK_SHIFT;
> +	unsigned long i;
> +
> +	dc->bypass_num_pages = DIV_ROUND_UP(chunks, BCH_BYPASS_PAGE_COUNTERS);
> +	dc->bypass_pages = kcalloc(dc->bypass_num_pages,
> +				   sizeof(struct bch_bypass_page),
> +				   GFP_KERNEL);

Maybe you can try kvcalloc(), for dc->bypass_pages, the physically continous
pages are not mandatory.

> +	if (!dc->bypass_pages)
> +		return -ENOMEM;
> +
> +	for (i = 0; i < dc->bypass_num_pages; i++)
> +		spin_lock_init(&dc->bypass_pages[i].lock);
> +
> +	return 0;
> +}
> +
>  static int cached_dev_init(struct cached_dev *dc, unsigned int block_size)
>  {
>  	int ret;
> @@ -1447,6 +1473,10 @@ static int cached_dev_init(struct cached_dev *dc, unsigned int block_size)
>  	/* default to auto */
>  	dc->stop_when_cache_set_failed = BCH_CACHED_DEV_STOP_AUTO;
>  
> +	ret = bch_cached_dev_bypass_init(dc, bdev_nr_sectors(dc->bdev));
> +	if (ret)
> +		return ret;
> +
>  	bch_cached_dev_request_init(dc);
>  	bch_cached_dev_writeback_init(dc);
>  	return 0;
> diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
> index cfac56caa804..26b17edd5576 100644
> --- a/drivers/md/bcache/sysfs.c
> +++ b/drivers/md/bcache/sysfs.c
> @@ -95,6 +95,7 @@ read_attribute(feature_incompat);
>  
>  read_attribute(state);
>  read_attribute(cache_read_races);
> +read_attribute(bypass_tracking_alloc_fails);
>  read_attribute(reclaim);
>  read_attribute(reclaimed_journal_buckets);
>  read_attribute(flush_write);
> @@ -748,6 +749,9 @@ SHOW(__bch_cache_set)
>  	sysfs_print(cache_read_races,
>  		    atomic_long_read(&c->cache_read_races));
>  
> +	sysfs_print(bypass_tracking_alloc_fails,
> +		    atomic_long_read(&c->bypass_tracking_alloc_fails));
> +
>  	sysfs_print(reclaim,
>  		    atomic_long_read(&c->reclaim));
>  
> @@ -993,6 +997,7 @@ static struct attribute *bch_cache_set_internal_attrs[] = {
>  
>  	&sysfs_bset_tree_stats,
>  	&sysfs_cache_read_races,
> +	&sysfs_bypass_tracking_alloc_fails,
>  	&sysfs_reclaim,
>  	&sysfs_reclaimed_journal_buckets,
>  	&sysfs_flush_write,
> diff --git a/include/trace/events/bcache.h b/include/trace/events/bcache.h
> index 899fdacf57b9..b76bca0c5285 100644
> --- a/include/trace/events/bcache.h
> +++ b/include/trace/events/bcache.h
> @@ -120,6 +120,11 @@ DEFINE_EVENT(bcache_bio, bcache_bypass_congested,
>  	TP_ARGS(bio)
>  );
>  
> +DEFINE_EVENT(bcache_bio, bcache_read_bypass_race,
> +	TP_PROTO(struct bio *bio),
> +	TP_ARGS(bio)
> +);
> +
>  TRACE_EVENT(bcache_read,
>  	TP_PROTO(struct bio *bio, bool hit, bool bypass),
>  	TP_ARGS(bio, hit, bypass),

Nice patch, thoughful coding and careful testings.

Thanks.

Coly Li