Messages in this thread | | | Date | Fri, 21 Feb 2014 00:13:16 -0500 | From | Peter Hurley <> | Subject | Re: [PATCH 4/9] firewire: don't use PREPARE_DELAYED_WORK |
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On 02/20/2014 09:13 PM, Tejun Heo wrote: > On Thu, Feb 20, 2014 at 09:07:27PM -0500, Peter Hurley wrote: >> On 02/20/2014 08:59 PM, Tejun Heo wrote: >>> Hello, >>> >>> On Thu, Feb 20, 2014 at 08:44:46PM -0500, Peter Hurley wrote: >>>>> +static void fw_device_workfn(struct work_struct *work) >>>>> +{ >>>>> + struct fw_device *device = container_of(to_delayed_work(work), >>>>> + struct fw_device, work); >>>> >>>> I think this needs an smp_rmb() here. >>> >>> The patch is equivalent transformation and the whole thing is >>> guaranteed to have gone through pool->lock. No explicit rmb >>> necessary. >> >> The spin_unlock_irq(&pool->lock) only guarantees completion of >> memory operations _before_ the unlock; memory operations which occur >> _after_ the unlock may be speculated before the unlock. >> >> IOW, unlock is not a memory barrier for operations that occur after. > > It's not just unlock. It's lock / unlock pair on the same lock from > both sides. Nothing can sip through that.
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).
Regards, Peter Hurley
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