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

["Wu, Feng" <feng.wu@xxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Thursday, September 10, 2015 8:45 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 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.

If all the assignment to pi_block_cpu is with one lock held, I don't think
we need to above checkout, we can safely use .pi_block_cpu, right?

Thanks,
Feng

> 
> Jan


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