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Mon, 28 Apr 2025 16:17:03 +0000 (GMT) Message-ID: Date: Mon, 28 Apr 2025 21:47:02 +0530 Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V3 08/20] block: don't allow to switch elevator if updating nr_hw_queues is in-progress To: Ming Lei Cc: Jens Axboe , linux-block@vger.kernel.org, Shinichiro Kawasaki , =?UTF-8?Q?Thomas_Hellstr=C3=B6m?= , Christoph Hellwig References: <20250424152148.1066220-1-ming.lei@redhat.com> <20250424152148.1066220-9-ming.lei@redhat.com> <748f3e46-51c1-47f8-8614-64efb30dd4ff@linux.ibm.com> Content-Language: en-US From: Nilay Shroff In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: YOSMb-FGSvTS3TDWagMwwn46nxavAGI7 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwNDI4MDEzMiBTYWx0ZWRfX68sjcF0X/vOz nICHKk3nGwClvYoTb3nZt73Q+/42xlnLUq+0MdPsys491MHYg54y4+tMBAHlgTn7KWXwgOSWUWL rVsF3I8rz3rytPJ0jCmGb6rjejx5JPG9U+7uyrN4lVu0jbZPv6UfJtrRMbwnFs7RCZEexkZUeTJ EUWM52pniPbE5rUHFS75tydp1ir4rot68SEFKl9lqAAa7iALVAiopYuxzkS2jQ9rO58qzxnxtQD 9237KwX/nZpWA3taBpHOVv9QJC4KpQXxHZ0m7o26fQmbbYGRTzWDo2YH1twJhIOR6Izr2J0Lm02 CAOXCxK673MyovtzzKXqgkFgO1b4Cexew+avu5lkarVpP44Q2lvvNK4+ikGUxD8bcvYNd1NJwmX SbMM4EHvOHrnNVEpZZfKrCmVf1KZinjfYCArauL02MMIzS0kzLw5IWZIIv6FVkoKTKzj4RQ+ X-Proofpoint-GUID: YOSMb-FGSvTS3TDWagMwwn46nxavAGI7 X-Authority-Analysis: v=2.4 cv=DvxW+H/+ c=1 sm=1 tr=0 ts=680faa04 cx=c_pps a=5BHTudwdYE3Te8bg5FgnPg==:117 a=5BHTudwdYE3Te8bg5FgnPg==:17 a=IkcTkHD0fZMA:10 a=XR8D0OoHHMoA:10 a=VwQbUJbxAAAA:8 a=JF9118EUAAAA:8 a=20KFwNOVAAAA:8 a=zJtk1i2aUhdcUKMgbdYA:9 a=QEXdDO2ut3YA:10 a=xVlTc564ipvMDusKsbsT:22 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1099,Hydra:6.0.736,FMLib:17.12.80.40 definitions=2025-04-28_06,2025-04-24_02,2025-02-21_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 mlxscore=0 bulkscore=0 lowpriorityscore=0 malwarescore=0 spamscore=0 mlxlogscore=999 phishscore=0 impostorscore=0 suspectscore=0 adultscore=0 clxscore=1015 priorityscore=1501 classifier=spam authscore=0 authtc=n/a authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2504070000 definitions=main-2504280132 On 4/27/25 7:57 AM, Ming Lei wrote: > On Fri, Apr 25, 2025 at 06:18:33PM +0530, Nilay Shroff wrote: >> >> >> On 4/24/25 8:51 PM, Ming Lei wrote: >>> Elevator switch code is another `nr_hw_queue` reader in non-fast-IO code >>> path, so it can't be done if updating `nr_hw_queues` is in-progress. >>> >>> Take same approach with not allowing add/del disk when updating >>> nr_hw_queues is in-progress, by grabbing read lock of >>> set->update_nr_hwq_sema. >>> >>> Take the nested variant for avoiding the following false positive >>> splat[1], and this way is correct because: >>> >>> - the read lock in elv_iosched_store() is not overlapped with the read lock >>> in adding/deleting disk: >>> >>> - kobject attribute is only available after the kobject is added and >>> before it is deleted >>> >>> -> #4 (&q->q_usage_counter(queue){++++}-{0:0}: >>> -> #3 (&q->limits_lock){+.+.}-{4:4}: >>> -> #2 (&disk->open_mutex){+.+.}-{4:4}: >>> -> #1 (&set->update_nr_hwq_lock){.+.+}-{4:4}: >>> -> #0 (kn->active#103){++++}-{0:0}: >>> >>> Link: https://lore.kernel.org/linux-block/aAWv3NPtNIKKvJZc@fedora/ [1] >>> Reported-by: Shinichiro Kawasaki >>> Closes: https://lore.kernel.org/linux-block/mz4t4tlwiqjijw3zvqnjb7ovvvaegkqganegmmlc567tt5xj67@xal5ro544cnc/ >>> Signed-off-by: Ming Lei >>> --- >>> block/elevator.c | 3 +++ >>> 1 file changed, 3 insertions(+) >>> >>> diff --git a/block/elevator.c b/block/elevator.c >>> index 4400eb8fe54f..56da6ab7691a 100644 >>> --- a/block/elevator.c >>> +++ b/block/elevator.c >>> @@ -723,6 +723,7 @@ ssize_t elv_iosched_store(struct gendisk *disk, const char *buf, >>> int ret; >>> unsigned int memflags; >>> struct request_queue *q = disk->queue; >>> + struct blk_mq_tag_set *set = q->tag_set; >>> >>> /* >>> * If the attribute needs to load a module, do it before freezing the >>> @@ -734,6 +735,7 @@ ssize_t elv_iosched_store(struct gendisk *disk, const char *buf, >>> >>> elv_iosched_load_module(name); >>> >>> + down_read_nested(&set->update_nr_hwq_sema, 1); >> >> Why do we need to add nested read lock here? The lockdep splat[1] which >> you reported earlier is possibly due to the same reader lock being acquired >> recursively in elv_iosched_store and then elevator_change? > > The splat isn't related with the nested read lock. > > If you replace down_read_nested() with down_read(), the same splat can be > triggered again when running `blktests block/001`. > I couldn't recreate it on my setup using above blktests. >> >> On another note, if we suspect possible one-depth recursion for the same >> class of lock then then we should use SINGLE_DEPTH_NESTING (instead of using >> 1 here) for subclass. But still I am not clear why this lock needs nesting. > > It is just one false positive, because elv_iosched_store() won't happen > when adding disk. > Yes, but how could we avoid false positive? It's probably because of commit ffa1e7ada456 ("block: Make request_queue lockdep splats show up earlier"). How about adding manual dependency of fs-reclaim on freeze-lock after we add the disk. Currently that manual dependency is added in blk_alloc_queue. Thanks, --Nilay