[PATCH] workqueue: Provide one lock class key per work_on_cpu() callsite
From: Frederic Weisbecker
Date: Sun Sep 24 2023 - 11:07:15 EST
All callers of work_on_cpu() share the same lock class key for all the
functions queued. As a result the workqueue related locking scenario for
a function A may be spuriously accounted as an inversion against the
locking scenario of function B such as in the following model:
long A(void *arg)
{
mutex_lock(&mutex);
mutex_unlock(&mutex);
}
long B(void *arg)
{
}
void launchA(void)
{
work_on_cpu(0, A, NULL);
}
void launchB(void)
{
mutex_lock(&mutex);
work_on_cpu(1, B, NULL);
mutex_unlock(&mutex);
}
launchA and launchB running concurrently have no chance to deadlock.
However the above can be reported by lockdep as a possible locking
inversion because the works containing A() and B() are treated as
belonging to the same locking class.
The following shows an existing example of such a spurious lockdep splat:
======================================================
WARNING: possible circular locking dependency detected
6.6.0-rc1-00065-g934ebd6e5359 #35409 Not tainted
------------------------------------------------------
kworker/0:1/9 is trying to acquire lock:
ffffffff9bc72f30 (cpu_hotplug_lock){++++}-{0:0}, at: _cpu_down+0x57/0x2b0
but task is already holding lock:
ffff9e3bc0057e60 ((work_completion)(&wfc.work)){+.+.}-{0:0}, at: process_scheduled_works+0x216/0x500
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #2 ((work_completion)(&wfc.work)){+.+.}-{0:0}:
__flush_work+0x83/0x4e0
work_on_cpu+0x97/0xc0
rcu_nocb_cpu_offload+0x62/0xb0
rcu_nocb_toggle+0xd0/0x1d0
kthread+0xe6/0x120
ret_from_fork+0x2f/0x40
ret_from_fork_asm+0x1b/0x30
-> #1 (rcu_state.barrier_mutex){+.+.}-{3:3}:
__mutex_lock+0x81/0xc80
rcu_nocb_cpu_deoffload+0x38/0xb0
rcu_nocb_toggle+0x144/0x1d0
kthread+0xe6/0x120
ret_from_fork+0x2f/0x40
ret_from_fork_asm+0x1b/0x30
-> #0 (cpu_hotplug_lock){++++}-{0:0}:
__lock_acquire+0x1538/0x2500
lock_acquire+0xbf/0x2a0
percpu_down_write+0x31/0x200
_cpu_down+0x57/0x2b0
__cpu_down_maps_locked+0x10/0x20
work_for_cpu_fn+0x15/0x20
process_scheduled_works+0x2a7/0x500
worker_thread+0x173/0x330
kthread+0xe6/0x120
ret_from_fork+0x2f/0x40
ret_from_fork_asm+0x1b/0x30
other info that might help us debug this:
Chain exists of:
cpu_hotplug_lock --> rcu_state.barrier_mutex --> (work_completion)(&wfc.work)
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock((work_completion)(&wfc.work));
lock(rcu_state.barrier_mutex);
lock((work_completion)(&wfc.work));
lock(cpu_hotplug_lock);
*** DEADLOCK ***
2 locks held by kworker/0:1/9:
#0: ffff900481068b38 ((wq_completion)events){+.+.}-{0:0}, at: process_scheduled_works+0x212/0x500
#1: ffff9e3bc0057e60 ((work_completion)(&wfc.work)){+.+.}-{0:0}, at: process_scheduled_works+0x216/0x500
stack backtrace:
CPU: 0 PID: 9 Comm: kworker/0:1 Not tainted 6.6.0-rc1-00065-g934ebd6e5359 #35409
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.12.0-59-gc9ba5276e321-prebuilt.qemu.org 04/01/2014
Workqueue: events work_for_cpu_fn
Call Trace:
rcu-torture: rcu_torture_read_exit: Start of episode
<TASK>
dump_stack_lvl+0x4a/0x80
check_noncircular+0x132/0x150
__lock_acquire+0x1538/0x2500
lock_acquire+0xbf/0x2a0
? _cpu_down+0x57/0x2b0
percpu_down_write+0x31/0x200
? _cpu_down+0x57/0x2b0
_cpu_down+0x57/0x2b0
__cpu_down_maps_locked+0x10/0x20
work_for_cpu_fn+0x15/0x20
process_scheduled_works+0x2a7/0x500
worker_thread+0x173/0x330
? __pfx_worker_thread+0x10/0x10
kthread+0xe6/0x120
? __pfx_kthread+0x10/0x10
ret_from_fork+0x2f/0x40
? __pfx_kthread+0x10/0x10
ret_from_fork_asm+0x1b/0x30
</TASK
Fix this with providing one lock class key per work_on_cpu() caller.
Reported-and-tested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
---
include/linux/workqueue.h | 46 +++++++++++++++++++++++++++++++++------
kernel/workqueue.c | 20 ++++++++++-------
2 files changed, 51 insertions(+), 15 deletions(-)
diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index 1c1d06804d45..24b1e5070f4d 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -274,18 +274,16 @@ static inline unsigned int work_static(struct work_struct *work) { return 0; }
* to generate better code.
*/
#ifdef CONFIG_LOCKDEP
-#define __INIT_WORK(_work, _func, _onstack) \
+#define __INIT_WORK_KEY(_work, _func, _onstack, _key) \
do { \
- static struct lock_class_key __key; \
- \
__init_work((_work), _onstack); \
(_work)->data = (atomic_long_t) WORK_DATA_INIT(); \
- lockdep_init_map(&(_work)->lockdep_map, "(work_completion)"#_work, &__key, 0); \
+ lockdep_init_map(&(_work)->lockdep_map, "(work_completion)"#_work, (_key), 0); \
INIT_LIST_HEAD(&(_work)->entry); \
(_work)->func = (_func); \
} while (0)
#else
-#define __INIT_WORK(_work, _func, _onstack) \
+#define __INIT_WORK_KEY(_work, _func, _onstack, _key) \
do { \
__init_work((_work), _onstack); \
(_work)->data = (atomic_long_t) WORK_DATA_INIT(); \
@@ -294,12 +292,22 @@ static inline unsigned int work_static(struct work_struct *work) { return 0; }
} while (0)
#endif
+#define __INIT_WORK(_work, _func, _onstack) \
+ do { \
+ static __maybe_unused struct lock_class_key __key; \
+ \
+ __INIT_WORK_KEY(_work, _func, _onstack, &__key); \
+ } while (0)
+
#define INIT_WORK(_work, _func) \
__INIT_WORK((_work), (_func), 0)
#define INIT_WORK_ONSTACK(_work, _func) \
__INIT_WORK((_work), (_func), 1)
+#define INIT_WORK_ONSTACK_KEY(_work, _func, _key) \
+ __INIT_WORK_KEY((_work), (_func), 1, _key)
+
#define __INIT_DELAYED_WORK(_work, _func, _tflags) \
do { \
INIT_WORK(&(_work)->work, (_func)); \
@@ -693,8 +701,32 @@ static inline long work_on_cpu_safe(int cpu, long (*fn)(void *), void *arg)
return fn(arg);
}
#else
-long work_on_cpu(int cpu, long (*fn)(void *), void *arg);
-long work_on_cpu_safe(int cpu, long (*fn)(void *), void *arg);
+long work_on_cpu_key(int cpu, long (*fn)(void *),
+ void *arg, struct lock_class_key *key);
+/*
+ * A new key is defined for each caller to make sure the work
+ * associated with the function doesn't share its locking class.
+ */
+#define work_on_cpu(_cpu, _fn, _arg) \
+({ \
+ static struct lock_class_key __key; \
+ \
+ work_on_cpu_key(_cpu, _fn, _arg, &__key); \
+})
+
+long work_on_cpu_safe_key(int cpu, long (*fn)(void *),
+ void *arg, struct lock_class_key *key);
+
+/*
+ * A new key is defined for each caller to make sure the work
+ * associated with the function doesn't share its locking class.
+ */
+#define work_on_cpu_safe(_cpu, _fn, _arg) \
+({ \
+ static struct lock_class_key __key; \
+ \
+ work_on_cpu_safe_key(_cpu, _fn, _arg, &__key); \
+})
#endif /* CONFIG_SMP */
#ifdef CONFIG_FREEZER
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index c85825e17df8..4374239c68f3 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -5612,50 +5612,54 @@ static void work_for_cpu_fn(struct work_struct *work)
}
/**
- * work_on_cpu - run a function in thread context on a particular cpu
+ * work_on_cpu_key - run a function in thread context on a particular cpu
* @cpu: the cpu to run on
* @fn: the function to run
* @arg: the function arg
+ * @key: The lock class key for lock debugging purposes
*
* It is up to the caller to ensure that the cpu doesn't go offline.
* The caller must not hold any locks which would prevent @fn from completing.
*
* Return: The value @fn returns.
*/
-long work_on_cpu(int cpu, long (*fn)(void *), void *arg)
+long work_on_cpu_key(int cpu, long (*fn)(void *),
+ void *arg, struct lock_class_key *key)
{
struct work_for_cpu wfc = { .fn = fn, .arg = arg };
- INIT_WORK_ONSTACK(&wfc.work, work_for_cpu_fn);
+ INIT_WORK_ONSTACK_KEY(&wfc.work, work_for_cpu_fn, key);
schedule_work_on(cpu, &wfc.work);
flush_work(&wfc.work);
destroy_work_on_stack(&wfc.work);
return wfc.ret;
}
-EXPORT_SYMBOL_GPL(work_on_cpu);
+EXPORT_SYMBOL_GPL(work_on_cpu_key);
/**
- * work_on_cpu_safe - run a function in thread context on a particular cpu
+ * work_on_cpu_safe_key - run a function in thread context on a particular cpu
* @cpu: the cpu to run on
* @fn: the function to run
* @arg: the function argument
+ * @key: The lock class key for lock debugging purposes
*
* Disables CPU hotplug and calls work_on_cpu(). The caller must not hold
* any locks which would prevent @fn from completing.
*
* Return: The value @fn returns.
*/
-long work_on_cpu_safe(int cpu, long (*fn)(void *), void *arg)
+long work_on_cpu_safe_key(int cpu, long (*fn)(void *),
+ void *arg, struct lock_class_key *key)
{
long ret = -ENODEV;
cpus_read_lock();
if (cpu_online(cpu))
- ret = work_on_cpu(cpu, fn, arg);
+ ret = work_on_cpu_key(cpu, fn, arg, key);
cpus_read_unlock();
return ret;
}
-EXPORT_SYMBOL_GPL(work_on_cpu_safe);
+EXPORT_SYMBOL_GPL(work_on_cpu_safe_key);
#endif /* CONFIG_SMP */
#ifdef CONFIG_FREEZER
--
2.41.0