On Sun, Dec 08, 2024 at 03:41:18PM +0530, Nilay Shroff wrote:
>
>
> On 12/7/24 08:47, Ming Lei wrote:
> > On Sat, Dec 7, 2024 at 12:48 AM Nilay Shroff <nilay@xxxxxxxxxxxxx> wrote:
> >>
> >> For storing a value to a queue attribute, the queue_attr_store function
> >> first freezes the queue (->q_usage_counter(io)) and then acquire
> >> ->sysfs_lock. This seems not correct as the usual ordering should be to
> >> acquire ->sysfs_lock before freezing the queue. This incorrect ordering
> >> causes the following lockdep splat which we are able to reproduce always
> >> simply by accessing /sys/kernel/debug file using ls command:
> >>
> >> [ 57.597146] WARNING: possible circular locking dependency detected
> >> [ 57.597154] 6.12.0-10553-gb86545e02e8c #20 Tainted: G W
> >> [ 57.597162] ------------------------------------------------------
> >> [ 57.597168] ls/4605 is trying to acquire lock:
> >> [ 57.597176] c00000003eb56710 (&mm->mmap_lock){++++}-{4:4}, at: __might_fault+0x58/0xc0
> >> [ 57.597200]
> >> but task is already holding lock:
> >> [ 57.597207] c0000018e27c6810 (&sb->s_type->i_mutex_key#3){++++}-{4:4}, at: iterate_dir+0x94/0x1d4
> >> [ 57.597226]
> >> which lock already depends on the new lock.
> >>
> >> [ 57.597233]
> >> the existing dependency chain (in reverse order) is:
> >> [ 57.597241]
> >> -> #5 (&sb->s_type->i_mutex_key#3){++++}-{4:4}:
> >> [ 57.597255] down_write+0x6c/0x18c
> >> [ 57.597264] start_creating+0xb4/0x24c
> >> [ 57.597274] debugfs_create_dir+0x2c/0x1e8
> >> [ 57.597283] blk_register_queue+0xec/0x294
> >> [ 57.597292] add_disk_fwnode+0x2e4/0x548
> >> [ 57.597302] brd_alloc+0x2c8/0x338
> >> [ 57.597309] brd_init+0x100/0x178
> >> [ 57.597317] do_one_initcall+0x88/0x3e4
> >> [ 57.597326] kernel_init_freeable+0x3cc/0x6e0
> >> [ 57.597334] kernel_init+0x34/0x1cc
> >> [ 57.597342] ret_from_kernel_user_thread+0x14/0x1c
> >> [ 57.597350]
> >> -> #4 (&q->debugfs_mutex){+.+.}-{4:4}:
> >> [ 57.597362] __mutex_lock+0xfc/0x12a0
> >> [ 57.597370] blk_register_queue+0xd4/0x294
> >> [ 57.597379] add_disk_fwnode+0x2e4/0x548
> >> [ 57.597388] brd_alloc+0x2c8/0x338
> >> [ 57.597395] brd_init+0x100/0x178
> >> [ 57.597402] do_one_initcall+0x88/0x3e4
> >> [ 57.597410] kernel_init_freeable+0x3cc/0x6e0
> >> [ 57.597418] kernel_init+0x34/0x1cc
> >> [ 57.597426] ret_from_kernel_user_thread+0x14/0x1c
> >> [ 57.597434]
> >> -> #3 (&q->sysfs_lock){+.+.}-{4:4}:
> >> [ 57.597446] __mutex_lock+0xfc/0x12a0
> >> [ 57.597454] queue_attr_store+0x9c/0x110
> >> [ 57.597462] sysfs_kf_write+0x70/0xb0
> >> [ 57.597471] kernfs_fop_write_iter+0x1b0/0x2ac
> >> [ 57.597480] vfs_write+0x3dc/0x6e8
> >> [ 57.597488] ksys_write+0x84/0x140
> >> [ 57.597495] system_call_exception+0x130/0x360
> >> [ 57.597504] system_call_common+0x160/0x2c4
> >> [ 57.597516]
> >> -> #2 (&q->q_usage_counter(io)#21){++++}-{0:0}:
> >> [ 57.597530] __submit_bio+0x5ec/0x828
> >> [ 57.597538] submit_bio_noacct_nocheck+0x1e4/0x4f0
> >> [ 57.597547] iomap_readahead+0x2a0/0x448
> >> [ 57.597556] xfs_vm_readahead+0x28/0x3c
> >> [ 57.597564] read_pages+0x88/0x41c
> >> [ 57.597571] page_cache_ra_unbounded+0x1ac/0x2d8
> >> [ 57.597580] filemap_get_pages+0x188/0x984
> >> [ 57.597588] filemap_read+0x13c/0x4bc
> >> [ 57.597596] xfs_file_buffered_read+0x88/0x17c
> >> [ 57.597605] xfs_file_read_iter+0xac/0x158
> >> [ 57.597614] vfs_read+0x2d4/0x3b4
> >> [ 57.597622] ksys_read+0x84/0x144
> >> [ 57.597629] system_call_exception+0x130/0x360
> >> [ 57.597637] system_call_common+0x160/0x2c4
> >> [ 57.597647]
> >> -> #1 (mapping.invalidate_lock#2){++++}-{4:4}:
> >> [ 57.597661] down_read+0x6c/0x220
> >> [ 57.597669] filemap_fault+0x870/0x100c
> >> [ 57.597677] xfs_filemap_fault+0xc4/0x18c
> >> [ 57.597684] __do_fault+0x64/0x164
> >> [ 57.597693] __handle_mm_fault+0x1274/0x1dac
> >> [ 57.597702] handle_mm_fault+0x248/0x484
> >> [ 57.597711] ___do_page_fault+0x428/0xc0c
> >> [ 57.597719] hash__do_page_fault+0x30/0x68
> >> [ 57.597727] do_hash_fault+0x90/0x35c
> >> [ 57.597736] data_access_common_virt+0x210/0x220
> >> [ 57.597745] _copy_from_user+0xf8/0x19c
> >> [ 57.597754] sel_write_load+0x178/0xd54
> >> [ 57.597762] vfs_write+0x108/0x6e8
> >> [ 57.597769] ksys_write+0x84/0x140
> >> [ 57.597777] system_call_exception+0x130/0x360
> >> [ 57.597785] system_call_common+0x160/0x2c4
> >> [ 57.597794]
> >> -> #0 (&mm->mmap_lock){++++}-{4:4}:
> >> [ 57.597806] __lock_acquire+0x17cc/0x2330
> >> [ 57.597814] lock_acquire+0x138/0x400
> >> [ 57.597822] __might_fault+0x7c/0xc0
> >> [ 57.597830] filldir64+0xe8/0x390
> >> [ 57.597839] dcache_readdir+0x80/0x2d4
> >> [ 57.597846] iterate_dir+0xd8/0x1d4
> >> [ 57.597855] sys_getdents64+0x88/0x2d4
> >> [ 57.597864] system_call_exception+0x130/0x360
> >> [ 57.597872] system_call_common+0x160/0x2c4
> >> [ 57.597881]
> >> other info that might help us debug this:
> >>
> >> [ 57.597888] Chain exists of:
> >> &mm->mmap_lock --> &q->debugfs_mutex --> &sb->s_type->i_mutex_key#3
> >>
> >> [ 57.597905] Possible unsafe locking scenario:
> >>
> >> [ 57.597911] CPU0 CPU1
> >> [ 57.597917] ---- ----
> >> [ 57.597922] rlock(&sb->s_type->i_mutex_key#3);
> >> [ 57.597932] lock(&q->debugfs_mutex);
> >> [ 57.597940] lock(&sb->s_type->i_mutex_key#3);
> >> [ 57.597950] rlock(&mm->mmap_lock);
> >> [ 57.597958]
> >> *** DEADLOCK ***
> >>
> >> [ 57.597965] 2 locks held by ls/4605:
> >> [ 57.597971] #0: c0000000137c12f8 (&f->f_pos_lock){+.+.}-{4:4}, at: fdget_pos+0xcc/0x154
> >> [ 57.597989] #1: c0000018e27c6810 (&sb->s_type->i_mutex_key#3){++++}-{4:4}, at: iterate_dir+0x94/0x1d4
> >>
> >> Prevent the above lockdep warning by acquiring ->sysfs_lock before
> >> freezing the queue while storing a queue attribute in queue_attr_store
> >> function.
> >>
> >> Reported-by: kjain101@xxxxxxxxxx
> >> Fixes: af2814149883 ("block: freeze the queue in queue_attr_store")
> >> Tested-by: kjain101@xxxxxxxxxx
> >> Cc: hch@xxxxxx
> >> Cc: axboe@xxxxxxxxx
> >> Cc: ritesh.list@xxxxxxxxx
> >> Cc: ming.lei@xxxxxxxxxx
> >> Cc: gjoyce@xxxxxxxxxxxxx
> >> Signed-off-by: Nilay Shroff <nilay@xxxxxxxxxxxxx>
> >> ---
> >> block/blk-sysfs.c | 4 ++--
> >> 1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> >> index 4241aea84161..f648b112782f 100644
> >> --- a/block/blk-sysfs.c
> >> +++ b/block/blk-sysfs.c
> >> @@ -706,11 +706,11 @@ queue_attr_store(struct kobject *kobj, struct attribute *attr,
> >> if (entry->load_module)
> >> entry->load_module(disk, page, length);
> >>
> >> - blk_mq_freeze_queue(q);
> >> mutex_lock(&q->sysfs_lock);
> >> + blk_mq_freeze_queue(q);
> >> res = entry->store(disk, page, length);
> >> - mutex_unlock(&q->sysfs_lock);
> >> blk_mq_unfreeze_queue(q);
> >> + mutex_unlock(&q->sysfs_lock);
> >> return res;
> >
> > We freeze queue first in __blk_mq_update_nr_hw_queues() in which
> > q->sysfs_lock is acquired after the freezing.
> >
> > So this simple fix may trigger a new ABBA warning.
> >
> Oops! yes I agree.
>
> How about (in addition to the current patch) updating __blk_mq_update_nr_hw_queues()
> so that we acquire q->sysfs_lock before freezing the queue? In fact, I tried it (and
> ensured that __blk_mq_update_nr_hw_queues() is triggered so that lockdep can track lock
> state and its dependencies) and that appears to be working good - no more lockdep
> warning. For reference attached the diff, in case, you want to have a look. Please let
> me know.
I think the idea is good since we grab ->syfs_lock before freezing queue
for long time, and the lock order is changed just since af2814149883 ("block: freeze
the queue in queue_attr_store"). And no one should try to acquire
->syfs_lock in io code path.
Also the patch improves the naughty updating_hw_queues code path too.
Thanks,
Ming
