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workqueue: Document (some) memory-ordering properties of {queue,schedule}_work()
It's desirable to be able to rely on the following property: All stores preceding (in program order) a call to a successful queue_work() will be visible from the CPU which will execute the queued work by the time such work executes, e.g., { x is initially 0 } CPU0 CPU1 WRITE_ONCE(x, 1); [ "work" is being executed ] r0 = queue_work(wq, work); r1 = READ_ONCE(x); Forbids: r0 == true && r1 == 0 The current implementation of queue_work() provides such memory-ordering property: - In __queue_work(), the ->lock spinlock is acquired. - On the other side, in worker_thread(), this same ->lock is held when dequeueing work. So the locking ordering makes things work out. Add this property to the DocBook headers of {queue,schedule}_work(). Suggested-by: Paul E. McKenney <paulmck@kernel.org> Signed-off-by: Andrea Parri <parri.andrea@gmail.com> Acked-by: Paul E. McKenney <paulmck@kernel.org> Signed-off-by: Tejun Heo <tj@kernel.org>
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include/linux
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@ -487,6 +487,19 @@ extern void wq_worker_comm(char *buf, size_t size, struct task_struct *task);
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*
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*
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* We queue the work to the CPU on which it was submitted, but if the CPU dies
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* We queue the work to the CPU on which it was submitted, but if the CPU dies
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* it can be processed by another CPU.
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* it can be processed by another CPU.
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*
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* Memory-ordering properties: If it returns %true, guarantees that all stores
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* preceding the call to queue_work() in the program order will be visible from
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* the CPU which will execute @work by the time such work executes, e.g.,
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*
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* { x is initially 0 }
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*
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* CPU0 CPU1
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*
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* WRITE_ONCE(x, 1); [ @work is being executed ]
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* r0 = queue_work(wq, work); r1 = READ_ONCE(x);
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*
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* Forbids: r0 == true && r1 == 0
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*/
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*/
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static inline bool queue_work(struct workqueue_struct *wq,
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static inline bool queue_work(struct workqueue_struct *wq,
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struct work_struct *work)
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struct work_struct *work)
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@ -546,6 +559,9 @@ static inline bool schedule_work_on(int cpu, struct work_struct *work)
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* This puts a job in the kernel-global workqueue if it was not already
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* This puts a job in the kernel-global workqueue if it was not already
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* queued and leaves it in the same position on the kernel-global
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* queued and leaves it in the same position on the kernel-global
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* workqueue otherwise.
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* workqueue otherwise.
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*
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* Shares the same memory-ordering properties of queue_work(), cf. the
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* DocBook header of queue_work().
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*/
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*/
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static inline bool schedule_work(struct work_struct *work)
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static inline bool schedule_work(struct work_struct *work)
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{
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{
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