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block: Fix potential deadlock while freezing queue and acquiring sysf…
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…s_lock

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 torvalds#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]
               -> ChimeraOS#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]
               -> ChimeraOS#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]
               -> ChimeraOS#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]
               -> ChimeraOS#2 (&q->q_usage_counter(io)torvalds#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]
               -> ChimeraOS#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]  ChimeraOS#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. Later, we also found[1] another function __blk_mq_update_nr_
hw_queues where we first freeze queue and then acquire the ->sysfs_lock.
So we've also updated lock ordering in __blk_mq_update_nr_hw_queues
function and ensured that in all code paths we follow the correct lock
ordering i.e. acquire ->sysfs_lock before freezing the queue.

[1] https://lore.kernel.org/all/CAFj5m9Ke8+EHKQBs_Nk6hqd=LGXtk4mUxZUN5==ZcCjnZSBwHw@mail.gmail.com/

Reported-by: kjain@linux.ibm.com
Fixes: af28141 ("block: freeze the queue in queue_attr_store")
Tested-by: kjain@linux.ibm.com
Cc: hch@lst.de
Cc: axboe@kernel.dk
Cc: ritesh.list@gmail.com
Cc: ming.lei@redhat.com
Cc: gjoyce@linux.ibm.com
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20241210144222.1066229-1-nilay@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
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@shroffni @axboe
shroffni authored and axboe committed Dec 13, 2024
1 parent a6fe7b7 commit be26ba9
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Showing 3 changed files with 26 additions and 23 deletions.
16 changes: 6 additions & 10 deletions block/blk-mq-sysfs.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -275,15 +275,13 @@ void blk_mq_sysfs_unregister_hctxs(struct request_queue *q)
struct blk_mq_hw_ctx *hctx;
unsigned long i;

mutex_lock(&q->sysfs_dir_lock);
lockdep_assert_held(&q->sysfs_dir_lock);

if (!q->mq_sysfs_init_done)
goto unlock;
return;

queue_for_each_hw_ctx(q, hctx, i)
blk_mq_unregister_hctx(hctx);

unlock:
mutex_unlock(&q->sysfs_dir_lock);
}

int blk_mq_sysfs_register_hctxs(struct request_queue *q)
Expand All @@ -292,18 +290,16 @@ int blk_mq_sysfs_register_hctxs(struct request_queue *q)
unsigned long i;
int ret = 0;

mutex_lock(&q->sysfs_dir_lock);
lockdep_assert_held(&q->sysfs_dir_lock);

if (!q->mq_sysfs_init_done)
goto unlock;
return ret;

queue_for_each_hw_ctx(q, hctx, i) {
ret = blk_mq_register_hctx(hctx);
if (ret)
break;
}

unlock:
mutex_unlock(&q->sysfs_dir_lock);

return ret;
}
29 changes: 18 additions & 11 deletions block/blk-mq.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -4453,7 +4453,8 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
unsigned long i, j;

/* protect against switching io scheduler */
mutex_lock(&q->sysfs_lock);
lockdep_assert_held(&q->sysfs_lock);

for (i = 0; i < set->nr_hw_queues; i++) {
int old_node;
int node = blk_mq_get_hctx_node(set, i);
Expand Down Expand Up @@ -4486,7 +4487,6 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,

xa_for_each_start(&q->hctx_table, j, hctx, j)
blk_mq_exit_hctx(q, set, hctx, j);
mutex_unlock(&q->sysfs_lock);

/* unregister cpuhp callbacks for exited hctxs */
blk_mq_remove_hw_queues_cpuhp(q);
Expand Down Expand Up @@ -4518,10 +4518,14 @@ int blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,

xa_init(&q->hctx_table);

mutex_lock(&q->sysfs_lock);

blk_mq_realloc_hw_ctxs(set, q);
if (!q->nr_hw_queues)
goto err_hctxs;

mutex_unlock(&q->sysfs_lock);

INIT_WORK(&q->timeout_work, blk_mq_timeout_work);
blk_queue_rq_timeout(q, set->timeout ? set->timeout : 30 * HZ);

Expand All @@ -4540,6 +4544,7 @@ int blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
return 0;

err_hctxs:
mutex_unlock(&q->sysfs_lock);
blk_mq_release(q);
err_exit:
q->mq_ops = NULL;
Expand Down Expand Up @@ -4920,12 +4925,12 @@ static bool blk_mq_elv_switch_none(struct list_head *head,
return false;

/* q->elevator needs protection from ->sysfs_lock */
mutex_lock(&q->sysfs_lock);
lockdep_assert_held(&q->sysfs_lock);

/* the check has to be done with holding sysfs_lock */
if (!q->elevator) {
kfree(qe);
goto unlock;
goto out;
}

INIT_LIST_HEAD(&qe->node);
Expand All @@ -4935,9 +4940,7 @@ static bool blk_mq_elv_switch_none(struct list_head *head,
__elevator_get(qe->type);
list_add(&qe->node, head);
elevator_disable(q);
unlock:
mutex_unlock(&q->sysfs_lock);

out:
return true;
}

Expand Down Expand Up @@ -4966,11 +4969,9 @@ static void blk_mq_elv_switch_back(struct list_head *head,
list_del(&qe->node);
kfree(qe);

mutex_lock(&q->sysfs_lock);
elevator_switch(q, t);
/* drop the reference acquired in blk_mq_elv_switch_none */
elevator_put(t);
mutex_unlock(&q->sysfs_lock);
}

static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
Expand All @@ -4990,8 +4991,11 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
if (set->nr_maps == 1 && nr_hw_queues == set->nr_hw_queues)
return;

list_for_each_entry(q, &set->tag_list, tag_set_list)
list_for_each_entry(q, &set->tag_list, tag_set_list) {
mutex_lock(&q->sysfs_dir_lock);
mutex_lock(&q->sysfs_lock);
blk_mq_freeze_queue(q);
}
/*
* Switch IO scheduler to 'none', cleaning up the data associated
* with the previous scheduler. We will switch back once we are done
Expand Down Expand Up @@ -5047,8 +5051,11 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
list_for_each_entry(q, &set->tag_list, tag_set_list)
blk_mq_elv_switch_back(&head, q);

list_for_each_entry(q, &set->tag_list, tag_set_list)
list_for_each_entry(q, &set->tag_list, tag_set_list) {
blk_mq_unfreeze_queue(q);
mutex_unlock(&q->sysfs_lock);
mutex_unlock(&q->sysfs_dir_lock);
}

/* Free the excess tags when nr_hw_queues shrink. */
for (i = set->nr_hw_queues; i < prev_nr_hw_queues; i++)
Expand Down
4 changes: 2 additions & 2 deletions block/blk-sysfs.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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;
}

Expand Down

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