Re: [Xen-devel] [PATCH v5 1/4] VT-d PI: track the number of vcpus on pi blocking list

[Chao Gao <chao.gao@xxxxxxxxx>]

On Fri, Sep 01, 2017 at 02:24:08AM -0600, Jan Beulich wrote:
>>>> On 01.09.17 at 03:39, <chao.gao@xxxxxxxxx> wrote:
>> After thinking it again, I want to define the counter as
>> a unsigned int variable for the following reasion:
>> 1. It is definite that the counter is closely related with
>> list_add() and list_del(). If the list is protected by the
>> lock, it is straightforward that the counter is also protected
>> by the lock.
>> 2. In patch 3, althought there are some lock-less readers, we
>> will check the counter still meets our requirement with the lock
>> held. Thus, I don't think there is a racing issue.
>
>I think that's fine, but then you still don't need LOCKed accesses
>to the counter for updating it; write_atomic() will suffice afaict.

A stupid question.
Is it contradictory that you think the counter can be protected by
the lock while suggesting using write_atomic() instead of LOCKed
accesses?

updating the counter is always accompanied by updating list and updating
list should in locked region. I meaned things like:

spin_lock()
list_add()
counter++
spin_unlock()

However, I am afraid that not using LOCKed accesses but using
write_atomic() means something like (separating updating the counter
from updating the list I think is not good):

spin_lock()
list_add()
spin_unlock()
write_atomic()

And I think this version is:

spin_lock()
list_add()
add_sized()
spin_unlock()

Thanks
Chao

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