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Mon, 17 Nov 2025 21:02:45 -0800 (PST) Date: Mon, 17 Nov 2025 21:02:43 -0800 From: Mohamed Khalfella To: Waiman Long Cc: Ming Lei , 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: <20251118050243.GC2376676-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-kernel@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: <702d904c-2d9a-42b4-95b3-0fa43d91e673@redhat.com> On Mon 2025-11-17 23:35:56 -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 [1] > --- > 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 corr > ectly, > 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 or not. There is also the blk_mq_update_nr_hw_queues() > function that will acquire the write lock. > Assuming blk_mq_del_queue_tag_set(), blk_mq_add_queue_tag_set() and other readers run in parallel? Are we talking about potential starvation here? If yes, is this reader starvation or writer starvation?