Re: [PATCHv3 0/3] block: blk-rq-qos: replace static key with atomic bitop

[Nilay Shroff <nilay@linux.ibm.com>]

Hi Jens,

Just a gentle ping on this patchset. There have been a couple of recent
reports[1][2] highlighting this issue. Could you please consider pulling it?

[1] https://lore.kernel.org/all/4f2bd603-dbd9-434c-9dfe-f2d4f1becd82@linux.ibm.com/
[2] https://lore.kernel.org/all/36e552a1-8e8e-4b6f-894b-e7e04e17196e@linux.ibm.com/

Thanks,
--Nilay

On 8/14/25 1:54 PM, Nilay Shroff wrote:
> Hi,
> 
> This patchset replaces the use of a static key in the I/O path (rq_qos_
> xxx()) with an atomic queue flag (QUEUE_FLAG_QOS_ENABLED). This change
> is made to eliminate a potential deadlock introduced by the use of static
> keys in the blk-rq-qos infrastructure, as reported by lockdep during
> blktests block/005[1].
> 
> The original static key approach was introduced to avoid unnecessary
> dereferencing of q->rq_qos when no blk-rq-qos module (e.g., blk-wbt or
> blk-iolatency) is configured. While efficient, enabling a static key at
> runtime requires taking cpu_hotplug_lock and jump_label_mutex, which
> becomes problematic if the queue is already frozen — causing a reverse
> dependency on ->freeze_lock. This results in a lockdep splat indicating
> a potential deadlock.
> 
> To resolve this, we now gate q->rq_qos access with a q->queue_flags
> bitop (QUEUE_FLAG_QOS_ENABLED), avoiding the static key and the associated
> locking altogether.
> 
> I compared both static key and atomic bitop implementations using ftrace
> function graph tracer over ~50 invocations of rq_qos_issue() while ensuring
> blk-wbt/blk-iolatency were disabled (i.e., no QoS functionality). For
> easy comparision, I made rq_qos_issue() noinline. The comparision was
> made on PowerPC machine.
> 
> Static Key disabled (QoS is not configured):
> 5d0: 00 00 00 60     nop    # patched in by static key framework
> 5d4: 20 00 80 4e     blr    # return (branch to link register)
> 
> Only a nop and blr (branch to link register) are executed — very lightweight.
> 
> atomic bitop (QoS is not configured):
> 5d0: 20 00 23 e9     ld      r9,32(r3)     # load q->queue_flags
> 5d4: 00 80 29 71     andi.   r9,r9,32768   # check QUEUE_FLAG_QOS_ENABLED (bit 15)
> 5d8: 20 00 82 4d     beqlr                 # return if bit not set
> 
> This performs an ld and andi. before returning. Slightly more work,
> but q->queue_flags is typically hot in cache during I/O submission.
> 
> With Static Key (disabled):
> Duration (us): min=0.668 max=0.816 avg≈0.750
> 
> With atomic bitop QUEUE_FLAG_QOS_ENABLED (bit not set):
> Duration (us): min=0.684 max=0.834 avg≈0.759
> 
> As expected, both versions are almost similar in cost. The added latency
> from an extra ld and andi. is in the range of ~9ns.
> 
> There're three patches in the series:
> - First patch is a minor optimization which skips evaluating q->rq_qos
>   check in the re_qos_done_bio() as it's not needed.
> - Second patch fixes a subtle issue in rq_qos_del() to ensure that
>   we decrement the block_rq_qos static key in rq_qos_del() when a 
>   QoS policy is detached from the queue.
> - Third patch replaces usage of block_rq_qos static key with atomic flag
>   QUEUE_FLAG_QOS_ENABLED and thus helps break cpu_hotplug_lock depedency
>   on freeze_lock and that eventually help to fix the lockdep splat.
> 
> As usual, feedback and review comments are welcome!
> 
> [1] https://lore.kernel.org/linux-block/4fdm37so3o4xricdgfosgmohn63aa7wj3ua4e5vpihoamwg3ui@fq42f5q5t5ic/
> 
> Changes from v2:
>   - Added a change to skip the q->rq_qos check in rq_qos_done_bio().
>     This is now part of the first patch in this series. (Yu Kuai)
>   - Added a separate patch to ensure block_rq_qos is decremented when
>     detaching a QoS policy in rq_qos_del(). This is now the second 
>     patch in this series. (Yu Kuai)
>   - Folded the third patch from v2 into the new third patch, as patch
>     ordering changed (the second patch from v2 is now the third patch
>     in v3).
> 
> Link to v2: https://lore.kernel.org/all/20250805171749.3448694-1-nilay@linux.ibm.com/
> 
> Changes from v1:
>   - For debugging I made rq_qos_issue() noinline in my local workspace,
>     but then inadvertently it slipped through the patchset upstream. So
>     reverted it and made rq_qos_issue() inline again as earlier.
>   - Added Reported-by and Closes tags in the first patch, which I
>     obviously missed to add in the first version.
> 
> Link to v1: https://lore.kernel.org/all/20250804122125.3271397-1-nilay@linux.ibm.com/
> 
> Nilay Shroff (3):
>   block: skip q->rq_qos check in rq_qos_done_bio()
>   block: decrement block_rq_qos static key in rq_qos_del()
>   block: avoid cpu_hotplug_lock depedency on freeze_lock
> 
>  block/blk-mq-debugfs.c |  1 +
>  block/blk-rq-qos.c     |  8 +++----
>  block/blk-rq-qos.h     | 48 +++++++++++++++++++++++++++---------------
>  include/linux/blkdev.h |  1 +
>  4 files changed, 37 insertions(+), 21 deletions(-)
>