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Subject[PATCH v3 23/24] kernel/workqueue: Use dynamic lockdep keys for workqueues
Date
Commit 87915adc3f0a ("workqueue: re-add lockdep dependencies for flushing")
improved deadlock checking in the workqueue implementation. Unfortunately
that patch also introduced a few false positive lockdep complaints. This
patch suppresses these false positives by allocating the workqueue mutex
lockdep key dynamically. An example of a false positive lockdep complaint
suppressed by this report can be found below. The root cause of the
lockdep complaint shown below is that the direct I/O code can call
alloc_workqueue() from inside a work item created by another
alloc_workqueue() call and that both workqueues share the same lockdep
key. This patch avoids that that lockdep complaint is triggered by
allocating the work queue lockdep keys dynamically. In other words, this
patch guarantees that a unique lockdep key is associated with each work
queue mutex.

======================================================
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: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
include/linux/workqueue.h | 28 +++---------------
kernel/workqueue.c | 60 +++++++++++++++++++++++++++++++++------
2 files changed, 55 insertions(+), 33 deletions(-)

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index 60d673e15632..d9a1a480e920 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -390,43 +390,23 @@ extern struct workqueue_struct *system_freezable_wq;
extern struct workqueue_struct *system_power_efficient_wq;
extern struct workqueue_struct *system_freezable_power_efficient_wq;

-extern struct workqueue_struct *
-__alloc_workqueue_key(const char *fmt, unsigned int flags, int max_active,
- struct lock_class_key *key, const char *lock_name, ...) __printf(1, 6);
-
/**
* alloc_workqueue - allocate a workqueue
* @fmt: printf format for the name of the workqueue
* @flags: WQ_* flags
* @max_active: max in-flight work items, 0 for default
- * @args...: args for @fmt
+ * remaining args: args for @fmt
*
* Allocate a workqueue with the specified parameters. For detailed
* information on WQ_* flags, please refer to
* Documentation/core-api/workqueue.rst.
*
- * The __lock_name macro dance is to guarantee that single lock_class_key
- * doesn't end up with different namesm, which isn't allowed by lockdep.
- *
* RETURNS:
* Pointer to the allocated workqueue on success, %NULL on failure.
*/
-#ifdef CONFIG_LOCKDEP
-#define alloc_workqueue(fmt, flags, max_active, args...) \
-({ \
- static struct lock_class_key __key; \
- const char *__lock_name; \
- \
- __lock_name = "(wq_completion)"#fmt#args; \
- \
- __alloc_workqueue_key((fmt), (flags), (max_active), \
- &__key, __lock_name, ##args); \
-})
-#else
-#define alloc_workqueue(fmt, flags, max_active, args...) \
- __alloc_workqueue_key((fmt), (flags), (max_active), \
- NULL, NULL, ##args)
-#endif
+struct workqueue_struct *alloc_workqueue(const char *fmt,
+ unsigned int flags,
+ int max_active, ...);

/**
* alloc_ordered_workqueue - allocate an ordered workqueue
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 0280deac392e..82e155f764b7 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -259,6 +259,8 @@ struct workqueue_struct {
struct wq_device *wq_dev; /* I: for sysfs interface */
#endif
#ifdef CONFIG_LOCKDEP
+ char *lock_name;
+ struct lock_class_key key;
struct lockdep_map lockdep_map;
#endif
char name[WQ_NAME_LEN]; /* I: workqueue name */
@@ -3314,11 +3316,50 @@ static int init_worker_pool(struct worker_pool *pool)
return 0;
}

+#ifdef CONFIG_LOCKDEP
+static void wq_init_lockdep(struct workqueue_struct *wq)
+{
+ char *lock_name;
+
+ lockdep_register_key(&wq->key);
+ lock_name = kasprintf(GFP_KERNEL, "%s%s", "(wq_completion)", wq->name);
+ if (!lock_name)
+ lock_name = wq->name;
+ lockdep_init_map(&wq->lockdep_map, lock_name, &wq->key, 0);
+}
+
+static void wq_unregister_lockdep(struct workqueue_struct *wq)
+{
+ lockdep_reset_lock(&wq->lockdep_map);
+ lockdep_unregister_key(&wq->key);
+}
+
+static void wq_free_lockdep(struct workqueue_struct *wq)
+{
+ if (wq->lock_name != wq->name)
+ kfree(wq->lock_name);
+}
+#else
+static void wq_init_lockdep(struct workqueue_struct *wq)
+{
+}
+
+static void wq_unregister_lockdep(struct workqueue_struct *wq)
+{
+}
+
+static void wq_free_lockdep(struct workqueue_struct *wq)
+{
+}
+#endif
+
static void rcu_free_wq(struct rcu_head *rcu)
{
struct workqueue_struct *wq =
container_of(rcu, struct workqueue_struct, rcu);

+ wq_free_lockdep(wq);
+
if (!(wq->flags & WQ_UNBOUND))
free_percpu(wq->cpu_pwqs);
else
@@ -3509,8 +3550,10 @@ static void pwq_unbound_release_workfn(struct work_struct *work)
* If we're the last pwq going away, @wq is already dead and no one
* is gonna access it anymore. Schedule RCU free.
*/
- if (is_last)
+ if (is_last) {
+ wq_unregister_lockdep(wq);
call_rcu_sched(&wq->rcu, rcu_free_wq);
+ }
}

/**
@@ -4044,11 +4087,9 @@ static int init_rescuer(struct workqueue_struct *wq)
return 0;
}

-struct workqueue_struct *__alloc_workqueue_key(const char *fmt,
- unsigned int flags,
- int max_active,
- struct lock_class_key *key,
- const char *lock_name, ...)
+struct workqueue_struct *alloc_workqueue(const char *fmt,
+ unsigned int flags,
+ int max_active, ...)
{
size_t tbl_size = 0;
va_list args;
@@ -4083,7 +4124,7 @@ struct workqueue_struct *__alloc_workqueue_key(const char *fmt,
goto err_free_wq;
}

- va_start(args, lock_name);
+ va_start(args, max_active);
vsnprintf(wq->name, sizeof(wq->name), fmt, args);
va_end(args);

@@ -4100,7 +4141,7 @@ struct workqueue_struct *__alloc_workqueue_key(const char *fmt,
INIT_LIST_HEAD(&wq->flusher_overflow);
INIT_LIST_HEAD(&wq->maydays);

- lockdep_init_map(&wq->lockdep_map, lock_name, key, 0);
+ wq_init_lockdep(wq);
INIT_LIST_HEAD(&wq->list);

if (alloc_and_link_pwqs(wq) < 0)
@@ -4138,7 +4179,7 @@ struct workqueue_struct *__alloc_workqueue_key(const char *fmt,
destroy_workqueue(wq);
return NULL;
}
-EXPORT_SYMBOL_GPL(__alloc_workqueue_key);
+EXPORT_SYMBOL_GPL(alloc_workqueue);

/**
* destroy_workqueue - safely terminate a workqueue
@@ -4191,6 +4232,7 @@ void destroy_workqueue(struct workqueue_struct *wq)
kthread_stop(wq->rescuer->task);

if (!(wq->flags & WQ_UNBOUND)) {
+ wq_unregister_lockdep(wq);
/*
* The base ref is never dropped on per-cpu pwqs. Directly
* schedule RCU free.
--
2.20.0.rc2.403.gdbc3b29805-goog
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