On Tue, Oct 23, 2012 at 10:46 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, 23 Oct 2012, Ming Lei wrote:
>
>> On Mon, Oct 22, 2012 at 10:33 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
>> >
>> > Tail recursion should be implemented as a loop, not as an explicit
>> > recursion. That is, the function should be:
>> >
>> > void pm_runtime_set_memalloc_noio(struct device *dev, bool enable)
>> > {
>> > do {
>> > dev->power.memalloc_noio_resume = enable;
>> >
>> > if (!enable) {
>> > /*
>> > * Don't clear the parent's flag if any of the
>> > * parent's children have their flag set.
>> > */
>> > if (device_for_each_child(dev->parent, NULL,
>> > dev_memalloc_noio))
>> > return;
>> > }
>> > dev = dev->parent;
>> > } while (dev);
>> > }
>>
>> OK, will take the non-recursion implementation for saving kernel
>> stack space.
>>
>> >
>> > except that you need to add locking, for two reasons:
>> >
>> > There's a race. What happens if another child sets the flag
>> > between the time device_for_each_child() runs and the next loop
>> > iteration?
>>
>> Yes, I know the race, and not adding a lock because the function
>> is mostly called in .probe() or .remove() callback and its parent's device
>> lock is held to avoid this race.
>>
>> Considered that it may be called in async probe() (scsi disk), one lock
>> is needed, the simplest way is to add a global lock. Any suggestion?
>
> No. Because of where you put the new flag, it must be protected by
> dev->power.lock. And this means the iterative implementation shown
> above can't be used as is. It will have to be more like this:
>
> void pm_runtime_set_memalloc_noio(struct device *dev, bool enable)
> {
> spin_lock_irq(&dev->power.lock);
> dev->power.memalloc_noio_resume = enable;
>
> while (dev->parent) {
> spin_unlock_irq(&dev->power.lock);
> dev = dev->parent;
>
> spin_lock_irq(&dev->power.lock);
> /*
> * Don't clear the parent's flag if any of the
> * parent's children have their flag set.
> */
> if (!enable && device_for_each_child(dev->parent, NULL,
s/dev->parent/dev
> dev_memalloc_noio))
> break;
> dev->power.memalloc_noio_resume = enable;
> }
> spin_unlock_irq(&dev->power.lock);
> }
With the problem of non-SMP-safe bitfields access, the power.lock should
be held, but that is not enough to prevent children from being probed or
disconnected. Looks another lock is still needed. I think a global lock
is OK in the infrequent path.
>
>> > Even without a race, access to bitfields is not SMP-safe
>> > without locking.
>>
>> You mean one ancestor device might not be in active when
>> one of its descendants is being probed or removed?
>
> No. Consider this example:
>
> struct foo {
> int a:1;
> int b:1;
> } x;
>
> Consider what happens if CPU 0 does "x.a = 1" at the same time as
> another CPU 1 does "x.b = 1". The compiler might produce object code
> looking like this for CPU 0:
>
> move x, reg1
> or 0x1, reg1
> move reg1, x
>
> and this for CPU 1:
>
> move x, reg2
> or 0x2, reg2
> move reg2, x
>
> With no locking, the two "or" instructions could execute
> simultaneously. What will the final value of x be?
>
> The two CPUs will interfere, even though they are touching different
> bitfields.
Got it, thanks for your detailed explanation.
Looks the problem is worse than above, not only bitfields are affected, the
adjacent fields might be involved too, see:
http://lwn.net/Articles/478657/
Thanks,
--
Ming Lei
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