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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