[PATCH] bfq: silence lockdep for bfqd/ioc lock inversion

[Khazhismel Kumykov <khazhy@google.com>]

lockdep warns of circular locking due to inversion between
bfq_insert_requests and bfq_exit_icq. If we end freeing a request when
merging, we *may* grab an ioc->lock if that request is the last refcount
to that ioc. bfq_bio_merge also potentially could have this ordering.
bfq_exit_icq, conversely, grabs bfqd but is always called with ioc->lock
held.

bfq_exit_icq may either be called from put_io_context_active with ioc
refcount raised, ioc_release_fn after the last refcount was already
dropped, or ioc_clear_queue, which is only called while queue is
quiesced or exiting, so the inverted orderings should never conflict.

Fixes: aee69d78dec0 ("block, bfq: introduce the BFQ-v0 I/O scheduler as
an extra scheduler")

Signed-off-by: Khazhismel Kumykov <khazhy@google.com>
---
 block/bfq-iosched.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

Noticed this lockdep running xfstests (generic/464) on top of a bfq
block device. I was also able to tease it out w/ binary trying to issue
requests that would end up merging while rapidly swapping the active
scheduler. As far as I could see, the deadlock would not actually occur,
so this patch opts to change lock class for the inverted case.

bfqd -> ioc :
[ 2995.524557] __lock_acquire+0x18f5/0x2660
[ 2995.524562] lock_acquire+0xb4/0x3a0
[ 2995.524565] _raw_spin_lock_irqsave+0x3f/0x60
[ 2995.524569] put_io_context+0x33/0x90.  -> ioc->lock grabbed
[ 2995.524573] blk_mq_free_request+0x51/0x140
[ 2995.524577] blk_put_request+0xe/0x10
[ 2995.524580] blk_attempt_req_merge+0x1d/0x30
[ 2995.524585] elv_attempt_insert_merge+0x56/0xa0
[ 2995.524590] blk_mq_sched_try_insert_merge+0x4b/0x60
[ 2995.524595] bfq_insert_requests+0x9e/0x18c0.    -> bfqd->lock grabbed
[ 2995.524598] blk_mq_sched_insert_requests+0xd6/0x2b0
[ 2995.524602] blk_mq_flush_plug_list+0x154/0x280
[ 2995.524606] blk_finish_plug+0x40/0x60
[ 2995.524609] ext4_writepages+0x696/0x1320
[ 2995.524614] do_writepages+0x1c/0x80
[ 2995.524621] __filemap_fdatawrite_range+0xd7/0x120
[ 2995.524625] sync_file_range+0xac/0xf0
[ 2995.524642] __x64_sys_sync_file_range+0x44/0x70
[ 2995.524646] do_syscall_64+0x31/0x40
[ 2995.524649] entry_SYSCALL_64_after_hwframe+0x44/0xae

ioc -> bfqd
[ 2995.524490] _raw_spin_lock_irqsave+0x3f/0x60
[ 2995.524498] bfq_exit_icq+0xa3/0xe0 -> bfqd->lock grabbed
[ 2995.524512] put_io_context_active+0x78/0xb0 -> ioc->lock grabbed
[ 2995.524516] exit_io_context+0x48/0x50
[ 2995.524519] do_exit+0x7e9/0xdd0
[ 2995.524526] do_group_exit+0x54/0xc0
[ 2995.524530] __x64_sys_exit_group+0x18/0x20
[ 2995.524534] do_syscall_64+0x31/0x40
[ 2995.524537] entry_SYSCALL_64_after_hwframe+0x44/0xae

Another trace where we grab ioc -> bfqd through bfq_exit_icq is when
changing elevator
               -> #1 (&(&bfqd->lock)->rlock){-.-.}:
[  646.890820]        lock_acquire+0x9b/0x140
[  646.894868]        _raw_spin_lock_irqsave+0x3b/0x50
[  646.899707]        bfq_exit_icq_bfqq+0x47/0x1f0
[  646.904196]        bfq_exit_icq+0x21/0x30
[  646.908160]        ioc_destroy_icq+0xf3/0x130
[  646.912466]        ioc_clear_queue+0xb8/0x140
[  646.916771]        elevator_switch_mq+0xa4/0x3c0
[  646.921333]        elevator_switch+0x5f/0x340

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 95586137194e..cb50ac0ffe80 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -5027,7 +5027,14 @@ static void bfq_exit_icq_bfqq(struct bfq_io_cq *bic, bool is_sync)
 	if (bfqq && bfqd) {
 		unsigned long flags;
 
-		spin_lock_irqsave(&bfqd->lock, flags);
+		/* bfq_exit_icq is usually called with ioc->lock held, which is
+		 * inverse order from elsewhere, which may grab ioc->lock
+		 * under bfqd->lock if we merge requests and drop the last ioc
+		 * refcount. Since exit_icq is either called with a refcount,
+		 * or with queue quiesced, use a differnet lock class to
+		 * silence lockdep
+		 */
+		spin_lock_irqsave_nested(&bfqd->lock, flags, 1);
 		bfqq->bic = NULL;
 		bfq_exit_bfqq(bfqd, bfqq);
 		bic_set_bfqq(bic, NULL, is_sync);
-- 
2.31.0.rc2.261.g7f71774620-goog