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Tue, 18 Nov 2025 09:59:21 -0800 (PST) Date: Tue, 18 Nov 2025 09:59:18 -0800 From: Mohamed Khalfella To: Ming Lei Cc: Waiman Long , Hillf Danton , Jens Axboe , linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/1] nvme: Convert tag_list mutex to rwsemaphore to avoid deadlock Message-ID: <20251118175918.GA337106-mkhalfella@purestorage.com> References: <20251117202414.4071380-1-mkhalfella@purestorage.com> <20251118013442.9414-1-hdanton@sina.com> <5db3bb06-0bf2-4ba3-b765-c217acda1b0c@redhat.com> <8b07bdd3-5779-4fe4-be05-a8c8efc89f9d@redhat.com> <702d904c-2d9a-42b4-95b3-0fa43d91e673@redhat.com> Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Tue 2025-11-18 12:50:52 +0800, Ming Lei wrote: > On Mon, Nov 17, 2025 at 11:35:56PM -0500, Waiman Long wrote: > > On 11/17/25 11:03 PM, Ming Lei wrote: > > > On Mon, Nov 17, 2025 at 10:42:04PM -0500, Waiman Long wrote: > > > > On 11/17/25 10:08 PM, Ming Lei wrote: > > > > > On Mon, Nov 17, 2025 at 09:24:21PM -0500, Waiman Long wrote: > > > > > > On 11/17/25 8:34 PM, Hillf Danton wrote: > > > > > > > On Mon, 17 Nov 2025 12:23:53 -0800 Mohamed Khalfella wrote: > > > > > > > > blk_mq_{add,del}_queue_tag_set() functions add and remove queues from > > > > > > > > tagset, the functions make sure that tagset and queues are marked as > > > > > > > > shared when two or more queues are attached to the same tagset. > > > > > > > > Initially a tagset starts as unshared and when the number of added > > > > > > > > queues reaches two, blk_mq_add_queue_tag_set() marks it as shared along > > > > > > > > with all the queues attached to it. When the number of attached queues > > > > > > > > drops to 1 blk_mq_del_queue_tag_set() need to mark both the tagset and > > > > > > > > the remaining queues as unshared. > > > > > > > > > > > > > > > > Both functions need to freeze current queues in tagset before setting on > > > > > > > > unsetting BLK_MQ_F_TAG_QUEUE_SHARED flag. While doing so, both functions > > > > > > > > hold set->tag_list_lock mutex, which makes sense as we do not want > > > > > > > > queues to be added or deleted in the process. This used to work fine > > > > > > > > until commit 98d81f0df70c ("nvme: use blk_mq_[un]quiesce_tagset") > > > > > > > > made the nvme driver quiesce tagset instead of quiscing individual > > > > > > > > queues. blk_mq_quiesce_tagset() does the job and quiesce the queues in > > > > > > > > set->tag_list while holding set->tag_list_lock also. > > > > > > > > > > > > > > > > This results in deadlock between two threads with these stacktraces: > > > > > > > > > > > > > > > > __schedule+0x48e/0xed0 > > > > > > > > schedule+0x5a/0xc0 > > > > > > > > schedule_preempt_disabled+0x11/0x20 > > > > > > > > __mutex_lock.constprop.0+0x3cc/0x760 > > > > > > > > blk_mq_quiesce_tagset+0x26/0xd0 > > > > > > > > nvme_dev_disable_locked+0x77/0x280 [nvme] > > > > > > > > nvme_timeout+0x268/0x320 [nvme] > > > > > > > > blk_mq_handle_expired+0x5d/0x90 > > > > > > > > bt_iter+0x7e/0x90 > > > > > > > > blk_mq_queue_tag_busy_iter+0x2b2/0x590 > > > > > > > > ? __blk_mq_complete_request_remote+0x10/0x10 > > > > > > > > ? __blk_mq_complete_request_remote+0x10/0x10 > > > > > > > > blk_mq_timeout_work+0x15b/0x1a0 > > > > > > > > process_one_work+0x133/0x2f0 > > > > > > > > ? mod_delayed_work_on+0x90/0x90 > > > > > > > > worker_thread+0x2ec/0x400 > > > > > > > > ? mod_delayed_work_on+0x90/0x90 > > > > > > > > kthread+0xe2/0x110 > > > > > > > > ? kthread_complete_and_exit+0x20/0x20 > > > > > > > > ret_from_fork+0x2d/0x50 > > > > > > > > ? kthread_complete_and_exit+0x20/0x20 > > > > > > > > ret_from_fork_asm+0x11/0x20 > > > > > > > > > > > > > > > > __schedule+0x48e/0xed0 > > > > > > > > schedule+0x5a/0xc0 > > > > > > > > blk_mq_freeze_queue_wait+0x62/0x90 > > > > > > > > ? destroy_sched_domains_rcu+0x30/0x30 > > > > > > > > blk_mq_exit_queue+0x151/0x180 > > > > > > > > disk_release+0xe3/0xf0 > > > > > > > > device_release+0x31/0x90 > > > > > > > > kobject_put+0x6d/0x180 > > > > > > > > nvme_scan_ns+0x858/0xc90 [nvme_core] > > > > > > > > ? nvme_scan_work+0x281/0x560 [nvme_core] > > > > > > > > nvme_scan_work+0x281/0x560 [nvme_core] > > > > > > > > process_one_work+0x133/0x2f0 > > > > > > > > ? mod_delayed_work_on+0x90/0x90 > > > > > > > > worker_thread+0x2ec/0x400 > > > > > > > > ? mod_delayed_work_on+0x90/0x90 > > > > > > > > kthread+0xe2/0x110 > > > > > > > > ? kthread_complete_and_exit+0x20/0x20 > > > > > > > > ret_from_fork+0x2d/0x50 > > > > > > > > ? kthread_complete_and_exit+0x20/0x20 > > > > > > > > ret_from_fork_asm+0x11/0x20 > > > > > > > > > > > > > > > > The top stacktrace is showing nvme_timeout() called to handle nvme > > > > > > > > command timeout. timeout handler is trying to disable the controller and > > > > > > > > as a first step, it needs to blk_mq_quiesce_tagset() to tell blk-mq not > > > > > > > > to call queue callback handlers. The thread is stuck waiting for > > > > > > > > set->tag_list_lock as it tires to walk the queues in set->tag_list. > > > > > > > > > > > > > > > > The lock is held by the second thread in the bottom stack which is > > > > > > > > waiting for one of queues to be frozen. The queue usage counter will > > > > > > > > drop to zero after nvme_timeout() finishes, and this will not happen > > > > > > > > because the thread will wait for this mutex forever. > > > > > > > > > > > > > > > > Convert set->tag_list_lock mutex to set->tag_list_rwsem rwsemaphore to > > > > > > > > avoid the deadlock. Update blk_mq_[un]quiesce_tagset() to take the > > > > > > > > semaphore for read since this is enough to guarantee no queues will be > > > > > > > > added or removed. Update blk_mq_{add,del}_queue_tag_set() to take the > > > > > > > > semaphore for write while updating set->tag_list and downgrade it to > > > > > > > > read while freezing the queues. It should be safe to update set->flags > > > > > > > > and hctx->flags while holding the semaphore for read since the queues > > > > > > > > are already frozen. > > > > > > > > > > > > > > > > Fixes: 98d81f0df70c ("nvme: use blk_mq_[un]quiesce_tagset") > > > > > > > > Signed-off-by: Mohamed Khalfella > > > > > > > > --- > > > > > > > > block/blk-mq-sysfs.c | 10 ++--- > > > > > > > > block/blk-mq.c | 95 +++++++++++++++++++++++------------------- > > > > > > > > include/linux/blk-mq.h | 4 +- > > > > > > > > 3 files changed, 58 insertions(+), 51 deletions(-) > > > > > > > > > > > > > > > > diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c > > > > > > > > index 58ec293373c6..f474781654fb 100644 > > > > > > > > --- a/block/blk-mq-sysfs.c > > > > > > > > +++ b/block/blk-mq-sysfs.c > > > > > > > > @@ -230,13 +230,13 @@ int blk_mq_sysfs_register(struct gendisk *disk) > > > > > > > > kobject_uevent(q->mq_kobj, KOBJ_ADD); > > > > > > > > - mutex_lock(&q->tag_set->tag_list_lock); > > > > > > > > + down_read(&q->tag_set->tag_list_rwsem); > > > > > > > > queue_for_each_hw_ctx(q, hctx, i) { > > > > > > > > ret = blk_mq_register_hctx(hctx); > > > > > > > > if (ret) > > > > > > > > goto out_unreg; > > > > > > > > } > > > > > > > > - mutex_unlock(&q->tag_set->tag_list_lock); > > > > > > > > + up_read(&q->tag_set->tag_list_rwsem); > > > > > > > > return 0; > > > > > > > > out_unreg: > > > > > > > > @@ -244,7 +244,7 @@ int blk_mq_sysfs_register(struct gendisk *disk) > > > > > > > > if (j < i) > > > > > > > > blk_mq_unregister_hctx(hctx); > > > > > > > > } > > > > > > > > - mutex_unlock(&q->tag_set->tag_list_lock); > > > > > > > > + up_read(&q->tag_set->tag_list_rwsem); > > > > > > > > kobject_uevent(q->mq_kobj, KOBJ_REMOVE); > > > > > > > > kobject_del(q->mq_kobj); > > > > > > > > @@ -257,10 +257,10 @@ void blk_mq_sysfs_unregister(struct gendisk *disk) > > > > > > > > struct blk_mq_hw_ctx *hctx; > > > > > > > > unsigned long i; > > > > > > > > - mutex_lock(&q->tag_set->tag_list_lock); > > > > > > > > + down_read(&q->tag_set->tag_list_rwsem); > > > > > > > > queue_for_each_hw_ctx(q, hctx, i) > > > > > > > > blk_mq_unregister_hctx(hctx); > > > > > > > > - mutex_unlock(&q->tag_set->tag_list_lock); > > > > > > > > + up_read(&q->tag_set->tag_list_rwsem); > > > > > > > > kobject_uevent(q->mq_kobj, KOBJ_REMOVE); > > > > > > > > kobject_del(q->mq_kobj); > > > > > > > > diff --git a/block/blk-mq.c b/block/blk-mq.c > > > > > > > > index d626d32f6e57..9211d32ce820 100644 > > > > > > > > --- a/block/blk-mq.c > > > > > > > > +++ b/block/blk-mq.c > > > > > > > > @@ -335,12 +335,12 @@ void blk_mq_quiesce_tagset(struct blk_mq_tag_set *set) > > > > > > > > { > > > > > > > > struct request_queue *q; > > > > > > > > - mutex_lock(&set->tag_list_lock); > > > > > > > > + down_read(&set->tag_list_rwsem); > > > > > > > > list_for_each_entry(q, &set->tag_list, tag_set_list) { > > > > > > > > if (!blk_queue_skip_tagset_quiesce(q)) > > > > > > > > blk_mq_quiesce_queue_nowait(q); > > > > > > > > } > > > > > > > > - mutex_unlock(&set->tag_list_lock); > > > > > > > > + up_read(&set->tag_list_rwsem); > > > > > > > > blk_mq_wait_quiesce_done(set); > > > > > > > > } > > > > > > > > @@ -350,12 +350,12 @@ void blk_mq_unquiesce_tagset(struct blk_mq_tag_set *set) > > > > > > > > { > > > > > > > > struct request_queue *q; > > > > > > > > - mutex_lock(&set->tag_list_lock); > > > > > > > > + down_read(&set->tag_list_rwsem); > > > > > > > > list_for_each_entry(q, &set->tag_list, tag_set_list) { > > > > > > > > if (!blk_queue_skip_tagset_quiesce(q)) > > > > > > > > blk_mq_unquiesce_queue(q); > > > > > > > > } > > > > > > > > - mutex_unlock(&set->tag_list_lock); > > > > > > > > + up_read(&set->tag_list_rwsem); > > > > > > > > } > > > > > > > > EXPORT_SYMBOL_GPL(blk_mq_unquiesce_tagset); > > > > > > > > @@ -4274,56 +4274,63 @@ static void queue_set_hctx_shared(struct request_queue *q, bool shared) > > > > > > > > } > > > > > > > > } > > > > > > > > -static void blk_mq_update_tag_set_shared(struct blk_mq_tag_set *set, > > > > > > > > - bool shared) > > > > > > > > -{ > > > > > > > > - struct request_queue *q; > > > > > > > > - unsigned int memflags; > > > > > > > > - > > > > > > > > - lockdep_assert_held(&set->tag_list_lock); > > > > > > > > - > > > > > > > > - list_for_each_entry(q, &set->tag_list, tag_set_list) { > > > > > > > > - memflags = blk_mq_freeze_queue(q); > > > > > > > > - queue_set_hctx_shared(q, shared); > > > > > > > > - blk_mq_unfreeze_queue(q, memflags); > > > > > > > > - } > > > > > > > > -} > > > > > > > > - > > > > > > > > static void blk_mq_del_queue_tag_set(struct request_queue *q) > > > > > > > > { > > > > > > > > struct blk_mq_tag_set *set = q->tag_set; > > > > > > > > + struct request_queue *firstq; > > > > > > > > + unsigned int memflags; > > > > > > > > - mutex_lock(&set->tag_list_lock); > > > > > > > > + down_write(&set->tag_list_rwsem); > > > > > > > > list_del(&q->tag_set_list); > > > > > > > > - if (list_is_singular(&set->tag_list)) { > > > > > > > > - /* just transitioned to unshared */ > > > > > > > > - set->flags &= ~BLK_MQ_F_TAG_QUEUE_SHARED; > > > > > > > > - /* update existing queue */ > > > > > > > > - blk_mq_update_tag_set_shared(set, false); > > > > > > > > + if (!list_is_singular(&set->tag_list)) { > > > > > > > > + up_write(&set->tag_list_rwsem); > > > > > > > > + goto out; > > > > > > > > } > > > > > > > > - mutex_unlock(&set->tag_list_lock); > > > > > > > > + > > > > > > > > + /* > > > > > > > > + * Transitioning the remaining firstq to unshared. > > > > > > > > + * Also, downgrade the semaphore to avoid deadlock > > > > > > > > + * with blk_mq_quiesce_tagset() while waiting for > > > > > > > > + * firstq to be frozen. > > > > > > > > + */ > > > > > > > > + set->flags &= ~BLK_MQ_F_TAG_QUEUE_SHARED; > > > > > > > > + downgrade_write(&set->tag_list_rwsem); > > > > > > > If the first lock waiter is for write, it could ruin your downgrade trick. > > > > > If the 1st waiter is for WEITE, rwsem_mark_wake() simply returns and grants > > > > > read lock to this caller, meantime wakes up nothing. > > > > > > > > > > That is exactly what this use case expects, so can you explain in detail why > > > > > `it could ruin your downgrade trick`? > > > > > > > > > > > That is true. The downgrade will wake up all the waiting readers at the > > > > > > front of the wait queue, but if there is one or more writers in the mix. The > > > > > > wakeup will stop when the first writer is hit and all the readers after that > > > > > > will not be woken up. > > > > > So waiters for WRITE won't be waken up by downgrade_write() if I understand correctly, > > > > > and rwsem_downgrade_wake() documents this behavior too. > > > > > > > > > > > We can theoretically provide a downgrade variant that wakes up all the > > > > > > readers if it is a useful feature. > > > > > The following up_read() in this code block will wake up the waiter for > > > > > WRITE, which finally wakes up other waiters for READ, then I am confused > > > > > what is the problem with this usage? > > > > I am just referring to the fact that not all the readers may be woken up. So > > > > if the deadlock is caused by one of those readers that is not woken up, it > > > > can be a problem. I haven't analyzed the deadlock scenario in detail to see > > > > if that is really the case. It is up to you and others who are more familiar > > > > with this code base to figure this out. > > > Follows the code base, which isn't special compared with other > > > downgrade_write() usages: > > > > > > blk_mq_del_queue_tag_set()/blk_mq_add_queue_tag_set(): > > > > > > down_write(&set->tag_list_rwsem); > > > ... > > > downgrade_write(&set->tag_list_rwsem); > > > ... > > > up_read(&set->tag_list_rwsem); > > > > > > All others are readers: > > > > > > down_read(&set->tag_list_rwsem); > > > ... > > > up_read(&set->tag_list_rwsem); > > > > > > > > > You mentioned reader may not be waken up in case of r/w mixed waiters, but I > > > don't see it is possible. > > > > I don't know if concurrent calls to blk_mq_del_queue_tag_set() is possible > > Both blk_mq_del_queue_tag_set() and blk_mq_add_queue_tag_set() can be > called concurrently with same `struct blk_mq_tag_set *` and different > 'struct request_queue *'. > > > or not. There is also the blk_mq_update_nr_hw_queues() function that will > > acquire the write lock. > > blk_mq_update_nr_hw_queues() doesn't add/remove queue to set->tag_list, so > I guess read lock may be enough. The reason blk_mq_update_nr_hw_queues() takes write lock is that it can change q->hctx_table. We do not want to miss hcxt in queue_set_hctx_shared() called from blk_mq_{add,del}_queue_tag_set(). > > Also blk_mq_update_nr_hw_queues() can be run concurrently with the above > two helpers. > > > Thanks, > Ming >