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Subject[PATCH 3/3] kernel/workqueue: Suppress a false positive lockdep complaint
Date
It can happen that the direct I/O queue creates and destroys an empty
workqueue from inside a work function. Avoid that this triggers the
false positive lockdep complaint shown below.

======================================================
WARNING: possible circular locking dependency detected
4.19.0-dbg+ #1 Not tainted
------------------------------------------------------
fio/4129 is trying to acquire lock:
00000000a01cfe1a ((wq_completion)"dio/%s"sb->s_id){+.+.}, at: flush_workqueue+0xd0/0x970

but task is already holding lock:
00000000a0acecf9 (&sb->s_type->i_mutex_key#14){+.+.}, at: ext4_file_write_iter+0x154/0x710

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #2 (&sb->s_type->i_mutex_key#14){+.+.}:
down_write+0x3d/0x80
__generic_file_fsync+0x77/0xf0
ext4_sync_file+0x3c9/0x780
vfs_fsync_range+0x66/0x100
dio_complete+0x2f5/0x360
dio_aio_complete_work+0x1c/0x20
process_one_work+0x481/0x9f0
worker_thread+0x63/0x5a0
kthread+0x1cf/0x1f0
ret_from_fork+0x24/0x30

-> #1 ((work_completion)(&dio->complete_work)){+.+.}:
process_one_work+0x447/0x9f0
worker_thread+0x63/0x5a0
kthread+0x1cf/0x1f0
ret_from_fork+0x24/0x30

-> #0 ((wq_completion)"dio/%s"sb->s_id){+.+.}:
lock_acquire+0xc5/0x200
flush_workqueue+0xf3/0x970
drain_workqueue+0xec/0x220
destroy_workqueue+0x23/0x350
sb_init_dio_done_wq+0x6a/0x80
do_blockdev_direct_IO+0x1f33/0x4be0
__blockdev_direct_IO+0x79/0x86
ext4_direct_IO+0x5df/0xbb0
generic_file_direct_write+0x119/0x220
__generic_file_write_iter+0x131/0x2d0
ext4_file_write_iter+0x3fa/0x710
aio_write+0x235/0x330
io_submit_one+0x510/0xeb0
__x64_sys_io_submit+0x122/0x340
do_syscall_64+0x71/0x220
entry_SYSCALL_64_after_hwframe+0x49/0xbe

other info that might help us debug this:

Chain exists of:
(wq_completion)"dio/%s"sb->s_id --> (work_completion)(&dio->complete_work) --> &sb->s_type->i_mutex_key#14

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&sb->s_type->i_mutex_key#14);
lock((work_completion)(&dio->complete_work));
lock(&sb->s_type->i_mutex_key#14);
lock((wq_completion)"dio/%s"sb->s_id);

*** DEADLOCK ***

1 lock held by fio/4129:
#0: 00000000a0acecf9 (&sb->s_type->i_mutex_key#14){+.+.}, at: ext4_file_write_iter+0x154/0x710

stack backtrace:
CPU: 3 PID: 4129 Comm: fio Not tainted 4.19.0-dbg+ #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
Call Trace:
dump_stack+0x86/0xc5
print_circular_bug.isra.32+0x20a/0x218
__lock_acquire+0x1c68/0x1cf0
lock_acquire+0xc5/0x200
flush_workqueue+0xf3/0x970
drain_workqueue+0xec/0x220
destroy_workqueue+0x23/0x350
sb_init_dio_done_wq+0x6a/0x80
do_blockdev_direct_IO+0x1f33/0x4be0
__blockdev_direct_IO+0x79/0x86
ext4_direct_IO+0x5df/0xbb0
generic_file_direct_write+0x119/0x220
__generic_file_write_iter+0x131/0x2d0
ext4_file_write_iter+0x3fa/0x710
aio_write+0x235/0x330
io_submit_one+0x510/0xeb0
__x64_sys_io_submit+0x122/0x340
do_syscall_64+0x71/0x220
entry_SYSCALL_64_after_hwframe+0x49/0xbe

Cc: Johannes Berg <johannes.berg@intel.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
include/linux/workqueue.h | 1 +
kernel/workqueue.c | 6 +++++-
2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index 60d673e15632..375ec764f148 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -344,6 +344,7 @@ enum {
__WQ_ORDERED = 1 << 17, /* internal: workqueue is ordered */
__WQ_LEGACY = 1 << 18, /* internal: create*_workqueue() */
__WQ_ORDERED_EXPLICIT = 1 << 19, /* internal: alloc_ordered_workqueue() */
+ __WQ_HAS_BEEN_USED = 1 << 20, /* internal: work has been queued */

WQ_MAX_ACTIVE = 512, /* I like 512, better ideas? */
WQ_MAX_UNBOUND_PER_CPU = 4, /* 4 * #cpus for unbound wq */
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index fc9129d5909e..0ef275fe526c 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1383,6 +1383,10 @@ static void __queue_work(int cpu, struct workqueue_struct *wq,
if (unlikely(wq->flags & __WQ_DRAINING) &&
WARN_ON_ONCE(!is_chained_work(wq)))
return;
+
+ if (!(wq->flags & __WQ_HAS_BEEN_USED))
+ wq->flags |= __WQ_HAS_BEEN_USED;
+
retry:
if (req_cpu == WORK_CPU_UNBOUND)
cpu = wq_select_unbound_cpu(raw_smp_processor_id());
@@ -2889,7 +2893,7 @@ static bool start_flush_work(struct work_struct *work, struct wq_barrier *barr,
* workqueues the deadlock happens when the rescuer stalls, blocking
* forward progress.
*/
- if (!from_cancel &&
+ if (!from_cancel && (pwq->wq->flags & __WQ_HAS_BEEN_USED) &&
(pwq->wq->saved_max_active == 1 || pwq->wq->rescuer)) {
lock_acquire_exclusive(&pwq->wq->lockdep_map, 0, 0, NULL,
_THIS_IP_);
--
2.19.1.568.g152ad8e336-goog
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