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Subject[PATCH AUTOSEL for 4.9 151/190] block/mq: Cure cpu hotplug lock inversion
Date
From: Peter Zijlstra <peterz@infradead.org>

[ Upstream commit eabe06595d62cfa9278e2cd012df614bc68a7042 ]

By poking at /debug/sched_features I triggered the following splat:

[] ======================================================
[] WARNING: possible circular locking dependency detected
[] 4.11.0-00873-g964c8b7-dirty #694 Not tainted
[] ------------------------------------------------------
[] bash/2109 is trying to acquire lock:
[] (cpu_hotplug_lock.rw_sem){++++++}, at: [<ffffffff8120cb8b>] static_key_slow_dec+0x1b/0x50
[]
[] but task is already holding lock:
[] (&sb->s_type->i_mutex_key#4){+++++.}, at: [<ffffffff81140216>] sched_feat_write+0x86/0x170
[]
[] which lock already depends on the new lock.
[]
[]
[] the existing dependency chain (in reverse order) is:
[]
[] -> #2 (&sb->s_type->i_mutex_key#4){+++++.}:
[] lock_acquire+0x100/0x210
[] down_write+0x28/0x60
[] start_creating+0x5e/0xf0
[] debugfs_create_dir+0x13/0x110
[] blk_mq_debugfs_register+0x21/0x70
[] blk_mq_register_dev+0x64/0xd0
[] blk_register_queue+0x6a/0x170
[] device_add_disk+0x22d/0x440
[] loop_add+0x1f3/0x280
[] loop_init+0x104/0x142
[] do_one_initcall+0x43/0x180
[] kernel_init_freeable+0x1de/0x266
[] kernel_init+0xe/0x100
[] ret_from_fork+0x31/0x40
[]
[] -> #1 (all_q_mutex){+.+.+.}:
[] lock_acquire+0x100/0x210
[] __mutex_lock+0x6c/0x960
[] mutex_lock_nested+0x1b/0x20
[] blk_mq_init_allocated_queue+0x37c/0x4e0
[] blk_mq_init_queue+0x3a/0x60
[] loop_add+0xe5/0x280
[] loop_init+0x104/0x142
[] do_one_initcall+0x43/0x180
[] kernel_init_freeable+0x1de/0x266
[] kernel_init+0xe/0x100
[] ret_from_fork+0x31/0x40

[] *** DEADLOCK ***
[]
[] 3 locks held by bash/2109:
[] #0: (sb_writers#11){.+.+.+}, at: [<ffffffff81292bcd>] vfs_write+0x17d/0x1a0
[] #1: (debugfs_srcu){......}, at: [<ffffffff8155a90d>] full_proxy_write+0x5d/0xd0
[] #2: (&sb->s_type->i_mutex_key#4){+++++.}, at: [<ffffffff81140216>] sched_feat_write+0x86/0x170
[]
[] stack backtrace:
[] CPU: 9 PID: 2109 Comm: bash Not tainted 4.11.0-00873-g964c8b7-dirty #694
[] Hardware name: Intel Corporation S2600GZ/S2600GZ, BIOS SE5C600.86B.02.02.0002.122320131210 12/23/2013
[] Call Trace:

[] lock_acquire+0x100/0x210
[] get_online_cpus+0x2a/0x90
[] static_key_slow_dec+0x1b/0x50
[] static_key_disable+0x20/0x30
[] sched_feat_write+0x131/0x170
[] full_proxy_write+0x97/0xd0
[] __vfs_write+0x28/0x120
[] vfs_write+0xb5/0x1a0
[] SyS_write+0x49/0xa0
[] entry_SYSCALL_64_fastpath+0x23/0xc2

This is because of the cpu hotplug lock rework. Break the chain at #1
by reversing the lock acquisition order. This way i_mutex_key#4 no
longer depends on cpu_hotplug_lock and things are good.

Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
block/blk-mq.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 10f8f94b7f20..c6572ffc1e87 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2014,15 +2014,15 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,

blk_mq_init_cpu_queues(q, set->nr_hw_queues);

- get_online_cpus();
mutex_lock(&all_q_mutex);
+ get_online_cpus();

list_add_tail(&q->all_q_node, &all_q_list);
blk_mq_add_queue_tag_set(set, q);
blk_mq_map_swqueue(q, cpu_online_mask);

- mutex_unlock(&all_q_mutex);
put_online_cpus();
+ mutex_unlock(&all_q_mutex);

return q;

--
2.14.1
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