On Fri, Feb 21, 2014 at 12:13:16AM -0500, Peter Hurley wrote: > CPU 0 | CPU 1 > | > INIT_WORK(fw_device_workfn) | > | > workfn = funcA | > queue_work_on() | > . | process_one_work() > . | .. > . | worker->current_func = work->func > . | > . | speculative load of workfn = funcA > . | . > workfn = funcB | . > queue_work_on() | . > local_irq_save() | . > test_and_set_bit() == 1 | . > | set_work_pool_and_clear_pending() > work is not queued | smp_wmb > funcB never runs | set_work_data() > | atomic_set() > | spin_unlock_irq() > | > | worker->current_func(work) @ fw_device_workfn > | workfn() @ funcA > > > The speculative load of workfn on CPU 1 is valid because no rmb will occur > between the load and the execution of workfn() on CPU 1. > > Thus funcB will never execute because, in this circumstance, a second > worker is not queued (because PENDING had not yet been cleared). There's no right or wrong execution. Executions of either funcA or funcB are correct results. The only memory ordering guarantee workqueue gives is that anything written before the work item is queued will be visible when that instance starts executing. When a work item is not queued, no ordering is guaranteed between the queueing attempt and the execution of the existing instance. We can add such guarantee, not sure how much it'd matter but it's not like it's gonna cost a lot either. This doesn't have much to do with the current series tho. In fact, PREPARE_WORK can't ever be made to give such guarantee. The function pointer has to fetched before clearing of PENDING. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
