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block: break pcpu_alloc_mutex dependency on freeze_lock

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While nr_hw_update allocates tagset tags it acquires ->pcpu_alloc_mutex
after ->freeze_lock is acquired or queue is frozen. This potentially
creates a circular dependency involving ->fs_reclaim if reclaim is
triggered simultaneously in a code path which first acquires ->pcpu_
alloc_mutex. As the queue is already frozen while nr_hw_queue update
allocates tagsets, the reclaim can't forward progress and thus it could
cause a potential deadlock as reported in lockdep splat[1].

Fix this by pre-allocating tagset tags before we freeze queue during
nr_hw_queue update. Later the allocated tagset tags could be safely
installed and used after queue is frozen.

Reported-by: Yi Zhang <yi.zhang@redhat.com>
Closes: https://lore.kernel.org/all/CAHj4cs8F=OV9s3La2kEQ34YndgfZP-B5PHS4Z8_b9euKG6J4mw@mail.gmail.com/ [1]
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Tested-by: Yi Zhang <yi.zhang@redhat.com>
Reviewed-by: Yu Kuai <yukuai@fnnas.com>
[axboe: fix brace style issue]
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Nilay Shroff 2026-03-01 18:29:43 +05:30 • committed by Jens Axboe
parent da46b5dfef
commit 539d1b47e9
1 changed files with 30 additions and 15 deletions
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45
block/blk-mq.c
View file

@ -4793,38 +4793,45 @@ static void blk_mq_update_queue_map(struct blk_mq_tag_set *set)
}
}
static int blk_mq_realloc_tag_set_tags(struct blk_mq_tag_set *set,
int new_nr_hw_queues)
static struct blk_mq_tags **blk_mq_prealloc_tag_set_tags(
struct blk_mq_tag_set *set,
int new_nr_hw_queues)
{
struct blk_mq_tags **new_tags;
int i;
if (set->nr_hw_queues >= new_nr_hw_queues)
goto done;
return NULL;
new_tags = kcalloc_node(new_nr_hw_queues, sizeof(struct blk_mq_tags *),
GFP_KERNEL, set->numa_node);
if (!new_tags)
return -ENOMEM;
return ERR_PTR(-ENOMEM);
if (set->tags)
memcpy(new_tags, set->tags, set->nr_hw_queues *
sizeof(*set->tags));
kfree(set->tags);
set->tags = new_tags;
for (i = set->nr_hw_queues; i < new_nr_hw_queues; i++) {
if (!__blk_mq_alloc_map_and_rqs(set, i)) {
while (--i >= set->nr_hw_queues)
__blk_mq_free_map_and_rqs(set, i);
return -ENOMEM;
if (blk_mq_is_shared_tags(set->flags)) {
new_tags[i] = set->shared_tags;
} else {
new_tags[i] = blk_mq_alloc_map_and_rqs(set, i,
set->queue_depth);
if (!new_tags[i])
goto out_unwind;
}
cond_resched();
}
done:
set->nr_hw_queues = new_nr_hw_queues;
return 0;
return new_tags;
out_unwind:
while (--i >= set->nr_hw_queues) {
if (!blk_mq_is_shared_tags(set->flags))
blk_mq_free_map_and_rqs(set, new_tags[i], i);
}
kfree(new_tags);
return ERR_PTR(-ENOMEM);
}
/*
@ -5113,6 +5120,7 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
unsigned int memflags;
int i;
struct xarray elv_tbl;
struct blk_mq_tags **new_tags;
bool queues_frozen = false;
lockdep_assert_held(&set->tag_list_lock);
@ -5147,11 +5155,18 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
if (blk_mq_elv_switch_none(q, &elv_tbl))
goto switch_back;
new_tags = blk_mq_prealloc_tag_set_tags(set, nr_hw_queues);
if (IS_ERR(new_tags))
goto switch_back;
list_for_each_entry(q, &set->tag_list, tag_set_list)
blk_mq_freeze_queue_nomemsave(q);
queues_frozen = true;
if (blk_mq_realloc_tag_set_tags(set, nr_hw_queues) < 0)
goto switch_back;
if (new_tags) {
kfree(set->tags);
set->tags = new_tags;
}
set->nr_hw_queues = nr_hw_queues;
fallback:
blk_mq_update_queue_map(set);