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[157.230.128.187]) by smtp.gmail.com with ESMTPSA id u18sm10164611pfl.40.2020.04.13.21.19.06 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 13 Apr 2020 21:19:08 -0700 (PDT) Received: by 42.do-not-panic.com (Postfix, from userid 1000) id 35461419C3; Tue, 14 Apr 2020 04:19:04 +0000 (UTC) From: Luis Chamberlain To: axboe@kernel.dk, viro@zeniv.linux.org.uk, bvanassche@acm.org, gregkh@linuxfoundation.org, rostedt@goodmis.org, mingo@redhat.com, jack@suse.cz, ming.lei@redhat.com, nstange@suse.de, akpm@linux-foundation.org Cc: mhocko@suse.com, yukuai3@huawei.com, linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Luis Chamberlain , Omar Sandoval , Hannes Reinecke , Michal Hocko Subject: [PATCH 5/5] block: revert back to synchronous request_queue removal Date: Tue, 14 Apr 2020 04:19:02 +0000 Message-Id: <20200414041902.16769-6-mcgrof@kernel.org> X-Mailer: git-send-email 2.23.0.rc1 In-Reply-To: <20200414041902.16769-1-mcgrof@kernel.org> References: <20200414041902.16769-1-mcgrof@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Commit dc9edc44de6c ("block: Fix a blk_exit_rl() regression") merged on v4.12 moved the work behind blk_release_queue() into a workqueue after a splat floated around which indicated some work on blk_release_queue() could sleep in blk_exit_rl(). This splat would be possible when a driver called blk_put_queue() or blk_cleanup_queue() (which calls blk_put_queue(= ) as its final call) from an atomic context. blk_put_queue() decrements the refcount for the request_queue kobject, and upon reaching 0 blk_release_queue() is called. Although blk_exit_rl() is now removed through commit db6d9952356 ("block: remove request_list code"), we reserve the right to be able to sleep within blk_release_queue() context. If you see no other way and *have* be in atomic context when you driver calls the last blk_put_queue() you can always just increase your block device's reference count with bdgrab() as this can be done in atomic context and the request_queue removal would be left to upper layers later. We document this bit of tribal knowledge as well now, and adjust kdoc format a bit. We revert back to synchronous request_queue removal because asynchronous removal creates a regression with expected userspace interaction with several drivers. An example is when removing the loopback driver and issues ioctl from userspace to do so, upon return and if successful one expects the device to be removed. Moving to asynchronous request_queue removal could have broken many scripts which relied on the removal to have been completed if there was no error. Using asynchronous request_queue removal however has helped us find other bugs, in the future we can test what could break with this arrangement by enabling CONFIG_DEBUG_KOBJECT_RELEASE. Cc: Bart Van Assche Cc: Omar Sandoval Cc: Hannes Reinecke Cc: Nicolai Stange Cc: Greg Kroah-Hartman Cc: Michal Hocko Cc: yu kuai Suggested-by: Nicolai Stange Fixes: dc9edc44de6c ("block: Fix a blk_exit_rl() regression") Signed-off-by: Luis Chamberlain --- block/blk-core.c | 19 ++++++++++++++++++- block/blk-sysfs.c | 38 +++++++++++++++++--------------------- include/linux/blkdev.h | 2 -- 3 files changed, 35 insertions(+), 24 deletions(-) diff --git a/block/blk-core.c b/block/blk-core.c index 5aaae7a1b338..8346c7c59ee6 100644 --- a/block/blk-core.c +++ b/block/blk-core.c @@ -301,6 +301,17 @@ void blk_clear_pm_only(struct request_queue *q) } EXPORT_SYMBOL_GPL(blk_clear_pm_only); =20 +/** + * blk_put_queue - decrement the request_queue refcount + * + * Decrements the refcount to the request_queue kobject, when this reach= es + * 0 we'll have blk_release_queue() called. You should avoid calling + * this function in atomic context but if you really have to ensure you + * first refcount the block device with bdgrab() / bdput() so that the + * last decrement happens in blk_cleanup_queue(). + * + * @q: the request_queue structure to decrement the refcount for + */ void blk_put_queue(struct request_queue *q) { kobject_put(&q->kobj); @@ -328,10 +339,16 @@ EXPORT_SYMBOL_GPL(blk_set_queue_dying); =20 /** * blk_cleanup_queue - shutdown a request queue - * @q: request queue to shutdown * * Mark @q DYING, drain all pending requests, mark @q DEAD, destroy and * put it. All future requests will be failed immediately with -ENODEV. + * + * You should not call this function in atomic context. If you need to + * refcount a request_queue in atomic context, instead refcount the + * block device with bdgrab() / bdput(). + * + * @q: request queue to shutdown + * */ void blk_cleanup_queue(struct request_queue *q) { diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c index 0285d67e1e4c..859911191ebc 100644 --- a/block/blk-sysfs.c +++ b/block/blk-sysfs.c @@ -860,22 +860,27 @@ static void blk_exit_queue(struct request_queue *q) bdi_put(q->backing_dev_info); } =20 - /** - * __blk_release_queue - release a request queue - * @work: pointer to the release_work member of the request queue to be = released + * blk_release_queue - release a request queue + * + * This function is called as part of the process when a block device is= being + * unregistered. Releasing a request queue starts with blk_cleanup_queue= (), + * which set the appropriate flags and then calls blk_put_queue() as the= last + * step. blk_put_queue() decrements the reference counter of the request= queue + * and once the reference counter reaches zero, this function is called = to + * release all allocated resources of the request queue. * - * Description: - * This function is called when a block device is being unregistered= . The - * process of releasing a request queue starts with blk_cleanup_queu= e, which - * set the appropriate flags and then calls blk_put_queue, that decr= ements - * the reference counter of the request queue. Once the reference co= unter - * of the request queue reaches zero, blk_release_queue is called to= release - * all allocated resources of the request queue. + * This function can sleep, and so we must ensure that the very last + * blk_put_queue() is never called from atomic context. + * + * @kobj: pointer to a kobject, who's container is a request_queue */ -static void __blk_release_queue(struct work_struct *work) +static void blk_release_queue(struct kobject *kobj) { - struct request_queue *q =3D container_of(work, typeof(*q), release_work= ); + struct request_queue *q =3D + container_of(kobj, struct request_queue, kobj); + + might_sleep(); =20 if (test_bit(QUEUE_FLAG_POLL_STATS, &q->queue_flags)) blk_stat_remove_callback(q, q->poll_cb); @@ -905,15 +910,6 @@ static void __blk_release_queue(struct work_struct *= work) call_rcu(&q->rcu_head, blk_free_queue_rcu); } =20 -static void blk_release_queue(struct kobject *kobj) -{ - struct request_queue *q =3D - container_of(kobj, struct request_queue, kobj); - - INIT_WORK(&q->release_work, __blk_release_queue); - schedule_work(&q->release_work); -} - static const struct sysfs_ops queue_sysfs_ops =3D { .show =3D queue_attr_show, .store =3D queue_attr_store, diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h index cc43c8e6516c..81f7ddb1587e 100644 --- a/include/linux/blkdev.h +++ b/include/linux/blkdev.h @@ -582,8 +582,6 @@ struct request_queue { =20 size_t cmd_size; =20 - struct work_struct release_work; - #define BLK_MAX_WRITE_HINTS 5 u64 write_hints[BLK_MAX_WRITE_HINTS]; }; --=20 2.25.1