workqueue: Provide one lock class key per work_on_cpu() callsite

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@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index 1c1d06804d45099f8a3e59ac615e11f65f822370..24b1e5070f4d438fdfc0365336141d45fbb03d3d 100644 (file)
--- 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 a3522b70218d3a8e77c2db02c0ad3bc4ea366ca0..0f682da96e1c52ea42aa8a23590cf8448fa27334 100644 (file)
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -5622,50 +5622,54 @@ 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