kernel/linux.git/block/elevator.h, branch v6.19.11

block: fix race between wbt_enable_default and IO submission

2025-12-12T19:51:11+00:00

When wbt_enable_default() is moved out of queue freezing in elevator_change(), it can cause the wbt inflight counter to become negative (-1), leading to hung tasks in the writeback path. Tasks get stuck in wbt_wait() because the counter is in an inconsistent state. The issue occurs because wbt_enable_default() could race with IO submission, allowing the counter to be decremented before proper initialization. This manifests as: rq_wait[0]: inflight: -1 has_waiters: True rwb_enabled() checks the state, which can be updated exactly between wbt_wait() (rq_qos_throttle()) and wbt_track()(rq_qos_track()), then the inflight counter will become negative. And results in hung task warnings like: task:kworker/u24:39 state:D stack:0 pid:14767 Call Trace: rq_qos_wait+0xb4/0x150 wbt_wait+0xa9/0x100 __rq_qos_throttle+0x24/0x40 blk_mq_submit_bio+0x672/0x7b0 ... Fix this by: 1. Splitting wbt_enable_default() into: - __wbt_enable_default(): Returns true if wbt_init() should be called - wbt_enable_default(): Wrapper for existing callers (no init) - wbt_init_enable_default(): New function that checks and inits WBT 2. Using wbt_init_enable_default() in blk_register_queue() to ensure proper initialization during queue registration 3. Move wbt_init() out of wbt_enable_default() which is only for enabling disabled wbt from bfq and iocost, and wbt_init() isn't needed. Then the original lock warning can be avoided. 4. Removing the ELEVATOR_FLAG_ENABLE_WBT_ON_EXIT flag and its handling code since it's no longer needed This ensures WBT is properly initialized before any IO can be submitted, preventing the counter from going negative. Cc: Nilay Shroff Cc: Yu Kuai Cc: Guangwu Zhang Fixes: 78c271344b6f ("block: move wbt_enable_default() out of queue freezing from sched ->exit()") Signed-off-by: Ming Lei Reviewed-by: Nilay Shroff Signed-off-by: Jens Axboe

block: remove the declaration of elevator_init_mq function

2025-11-18T23:27:19+00:00

In commit 1e44bedbc921 ("block: unifying elevator change"), the elevator_init_mq function was deleted, but its declaration in elevator.h was overlooked. This patch fixes it. Signed-off-by: Chengkaitao Reviewed-by: Chaitanya Kulkarni Signed-off-by: Jens Axboe

block: use {alloc|free}_sched data methods

2025-11-13T16:27:49+00:00

The previous patch introduced ->alloc_sched_data and ->free_sched_data methods. This patch builds upon that by now using these methods during elevator switch and nr_hw_queue update. It's also ensured that scheduler-specific data is allocated and freed through the new callbacks outside of the ->freeze_lock and ->elevator_lock locking contexts, thereby preventing any dependency on pcpu_alloc_mutex. Reviewed-by: Ming Lei Reviewed-by: Yu Kuai Signed-off-by: Nilay Shroff Signed-off-by: Jens Axboe

block: introduce alloc_sched_data and free_sched_data elevator methods

2025-11-13T16:27:49+00:00

The recent lockdep splat [1] highlights a potential deadlock risk involving ->elevator_lock and ->freeze_lock dependencies on -pcpu_alloc_ mutex. The trace shows that the issue occurs when the Kyber scheduler allocates dynamic memory for its elevator data during initialization. To address this, introduce two new elevator operation callbacks: ->alloc_sched_data and ->free_sched_data. The subsequent patch would build upon these newly introduced methods to suppress lockdep splat[1]. [1] https://lore.kernel.org/all/CAGVVp+VNW4M-5DZMNoADp6o2VKFhi7KxWpTDkcnVyjO0=-D5+A@mail.gmail.com/ Signed-off-by: Nilay Shroff Reviewed-by: Ming Lei Signed-off-by: Jens Axboe

block: move elevator tags into struct elevator_resources

2025-11-13T16:27:49+00:00

This patch introduces a new structure, struct elevator_resources, to group together all elevator-related resources that share the same lifetime. As a first step, this change moves the elevator tag pointer from struct elv_change_ctx into the new struct elevator_resources. Additionally, rename blk_mq_alloc_sched_tags_batch() and blk_mq_free_sched_tags_batch() to blk_mq_alloc_sched_res_batch() and blk_mq_free_sched_res_batch(), respectively. Introduce two new wrapper helpers, blk_mq_alloc_sched_res() and blk_mq_free_sched_res(), around blk_mq_alloc_sched_tags() and blk_mq_free_sched_tags(). These changes pave the way for consolidating the allocation and freeing of elevator-specific resources into common helper functions. This refactoring improves encapsulation and prepares the code for future extensions, allowing additional elevator-specific data to be added to struct elevator_resources without cluttering struct elv_change_ctx. Subsequent patches will extend struct elevator_resources to include other elevator-related data. Reviewed-by: Ming Lei Reviewed-by: Yu Kuai Signed-off-by: Nilay Shroff Signed-off-by: Jens Axboe

block: unify elevator tags and type xarrays into struct elv_change_ctx

2025-11-13T16:27:49+00:00

Currently, the nr_hw_queues update path manages two disjoint xarrays — one for elevator tags and another for elevator type — both used during elevator switching. Maintaining these two parallel structures for the same purpose adds unnecessary complexity and potential for mismatched state. This patch unifies both xarrays into a single structure, struct elv_change_ctx, which holds all per-queue elevator change context. A single xarray, named elv_tbl, now maps each queue (q->id) in a tagset to its corresponding elv_change_ctx entry, encapsulating the elevator tags, type and name references. This unification simplifies the code, improves maintainability, and clarifies ownership of per-queue elevator state. Reviewed-by: Ming Lei Reviewed-by: Yu Kuai Signed-off-by: Nilay Shroff Signed-off-by: Jens Axboe

blk-mq: fix elevator depth_updated method

2025-09-05T19:52:52+00:00

Current depth_updated has some problems: 1) depth_updated() will be called for each hctx, while all elevators will update async_depth for the disk level, this is not related to hctx; 2) In blk_mq_update_nr_requests(), if previous hctx update succeed and this hctx update failed, q->nr_requests will not be updated, while async_depth is already updated with new nr_reqeuests in previous depth_updated(); 3) All elevators are using q->nr_requests to calculate async_depth now, however, q->nr_requests is still the old value when depth_updated() is called from blk_mq_update_nr_requests(); Those problems are first from error path, then mq-deadline, and recently for bfq and kyber, fix those problems by: - pass in request_queue instead of hctx; - move depth_updated() after q->nr_requests is updated in blk_mq_update_nr_requests(); - add depth_updated() call inside init_sched() method to initialize async_depth; - remove init_hctx() method for mq-deadline and bfq that is useless now; Fixes: 77f1e0a52d26 ("bfq: update internal depth state when queue depth changes") Fixes: 39823b47bbd4 ("block/mq-deadline: Fix the tag reservation code") Fixes: 42e6c6ce03fd ("lib/sbitmap: convert shallow_depth from one word to the whole sbitmap") Signed-off-by: Yu Kuai Reviewed-by: Hannes Reinecke Reviewed-by: Li Nan Reviewed-by: Nilay Shroff Link: https://lore.kernel.org/r/20250821060612.1729939-2-yukuai1@huaweicloud.com Signed-off-by: Jens Axboe

block: fix lockdep warning caused by lock dependency in elv_iosched_store

2025-07-30T12:20:51+00:00

Recent lockdep reports [1] have revealed a potential deadlock caused by a lock dependency between the percpu allocator lock and the elevator lock. This issue can be avoided by ensuring that the allocation and release of scheduler tags (sched_tags) are performed outside the elevator lock. Furthermore, the queue does not need to be remain frozen during these operations. To address this, move all sched_tags allocations and deallocations outside of both the ->elevator_lock and the ->freeze_lock. Since the lifetime of the elevator queue and its associated sched_tags is closely tied, the allocated sched_tags are now stored in the elevator queue structure. Then, during the actual elevator switch (which runs under ->freeze_lock and ->elevator_lock), the pre-allocated sched_tags are assigned to the appropriate q->hctx. Once the elevator switch is complete and the locks are released, the old elevator queue and its associated sched_tags are freed. This commit specifically addresses the allocation/deallocation of sched_ tags during elevator switching. Note that sched_tags may also be allocated in other contexts, such as during nr_hw_queues updates. Supporting that use case will require batch allocation/deallocation, which will be handled in a follow-up patch. This restructuring ensures that sched_tags memory management occurs entirely outside of the ->elevator_lock and ->freeze_lock context, eliminating the lock dependency problem seen during scheduler updates. [1] https://lore.kernel.org/all/0659ea8d-a463-47c8-9180-43c719e106eb@linux.ibm.com/ Reported-by: Stefan Haberland Closes: https://lore.kernel.org/all/0659ea8d-a463-47c8-9180-43c719e106eb@linux.ibm.com/ Reviewed-by: Ming Lei Reviewed-by: Christoph Hellwig Reviewed-by: Hannes Reinecke Signed-off-by: Nilay Shroff Link: https://lore.kernel.org/r/20250730074614.2537382-3-nilay@linux.ibm.com Signed-off-by: Jens Axboe

block: move elevator queue allocation logic into blk_mq_init_sched

2025-07-30T12:20:51+00:00

In preparation for allocating sched_tags before freezing the request queue and acquiring ->elevator_lock, move the elevator queue allocation logic from the elevator ops ->init_sched callback into blk_mq_init_sched. As elevator_alloc is now only invoked from block layer core, we don't need to export it, so unexport elevator_alloc function. This refactoring provides a centralized location for elevator queue initialization, which makes it easier to store pre-allocated sched_tags in the struct elevator_queue during later changes. Reviewed-by: Ming Lei Reviewed-by: Hannes Reinecke Reviewed-by: Christoph Hellwig Signed-off-by: Nilay Shroff Link: https://lore.kernel.org/r/20250730074614.2537382-2-nilay@linux.ibm.com Signed-off-by: Jens Axboe

block: move wbt_enable_default() out of queue freezing from sched ->exit()

2025-05-06T13:43:43+00:00

scheduler's ->exit() is called with queue frozen and elevator lock is held, and wbt_enable_default() can't be called with queue frozen, otherwise the following lockdep warning is triggered: #6 (&q->rq_qos_mutex){+.+.}-{4:4}: #5 (&eq->sysfs_lock){+.+.}-{4:4}: #4 (&q->elevator_lock){+.+.}-{4:4}: #3 (&q->q_usage_counter(io)#3){++++}-{0:0}: #2 (fs_reclaim){+.+.}-{0:0}: #1 (&sb->s_type->i_mutex_key#3){+.+.}-{4:4}: #0 (&q->debugfs_mutex){+.+.}-{4:4}: Fix the issue by moving wbt_enable_default() out of bfq's exit(), and call it from elevator_change_done(). Meantime add disk->rqos_state_mutex for covering wbt state change, which matches the purpose more than ->elevator_lock. Reviewed-by: Hannes Reinecke Reviewed-by: Nilay Shroff Signed-off-by: Ming Lei Reviewed-by: Christoph Hellwig Link: https://lore.kernel.org/r/20250505141805.2751237-26-ming.lei@redhat.com Signed-off-by: Jens Axboe