Re: [Xen-devel] [PATCH v6 16/18] vmx: Add some scheduler hooks for VT-d posted interrupts

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.09.15 at 14:34, <feng.wu@xxxxxxxxx> wrote:

> 
>> -----Original Message-----
>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: Thursday, September 10, 2015 6:01 PM
>> To: Wu, Feng
>> Cc: Andrew Cooper; Dario Faggioli; George Dunlap; Tian, Kevin;
>> xen-devel@xxxxxxxxxxxxx; Keir Fraser
>> Subject: RE: [PATCH v6 16/18] vmx: Add some scheduler hooks for VT-d posted
>> interrupts
>> 
>> >>> On 10.09.15 at 11:41, <feng.wu@xxxxxxxxx> wrote:
>> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> >> Sent: Thursday, September 10, 2015 5:26 PM
>> >> >>> On 10.09.15 at 10:59, <feng.wu@xxxxxxxxx> wrote:
>> >> > First, how to check it while waiting to acquire the lock .pi_block_cpu
>> >> > didn't change?
>> >>
>> >> Note the difference between "check while waiting" and "check that
>> >> while waiting": The former is indeed hard to implement, while the
>> >> latter is pretty straightforward (and we do so elsewhere).
>> >>
>> >> > Secondly, even if we can check it, what should we do if .pi_block_cpu
>> >> > is changed after acquiring the lock as I mentioned above?
>> >>
>> >> Drop the lock and start over. I.e. (taking your pseudo code)
>> >>
>> >> restart:
>> >>     local_pi_block_cpu = ...;
>> >>     bail-if-invalid (e.g. -1 in current model)
>> >>     spin_lock_irqsave(&per_cpu(, local_pi_block_cpu), flags);
>> >>     if(local_pi_block_cpu != actual_pi_block_cpu) {
>> >>         spin_unlock_irqrestore(&per_cpu(,local_pi_block_cpu), flags);
>> >>         goto restart;
>> >>     }
>> >
>> > Thanks a lot for showing me this pseudo code! My concern is if
>> > .pi_block_vcpu is changed to -1 at this point, it doesn't work.
>> > .pi_block_vcpu being -1 here means the vCPU is remove from
>> > the blocking list by others, then we cannot delete it again via
>> > list_del() here.
>> 
>> Did you miss the "bail-if-invalid" above?
> 
> I am sorry, do I miss something here? If .pi_block_cpu becomes
> -1 here (after the above 'if' statement is finished with
> local_pi_block_cpu == actual_pi_block_cpu ), how can "bail-if-invalid"
> above help?

The (obvious I thought) implication is that all assignments to
pi_block_cpu (along with all list manipulations) now need to happen
with the lock held.

Jan


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