Re: [Xen-devel] [PATCH for-4.12 V4] x86/altp2m: fix HVMOP_altp2m_set_domain_state race

[George Dunlap <George.Dunlap@xxxxxxxxxx>]

> On Feb 15, 2019, at 1:47 PM, Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx> wrote:
> 
> On 15/02/2019 13:37, George Dunlap wrote:
>> 
>>>> The one issue is that domain_pause_except_self() currently is actually a 
>>>> deadlock risk if two different vcpus start it at the same time.  I think 
>>>> the 
>>>> attached patch (compile-tested only) should fix this issue; after this 
>>>> patch 
>>>> you should be able to use domain_pause_except_self() in 
>>>> altp2m_set_domain_state instead.
>>> There's one thing I don't really like here, which is a result of the
>>> (necessary) re-use of the hypercall deadlock mutex: This
>>> certainly poses the risk of getting called from a context where
>>> the lock was already acquired. Therefore I'd like to suggest to
>>> use this lock in a recursive way (here and elsewhere).
> 
> I can't think of a usecase were we would want to tolerate recursion on
> the hypercall deadlock spinlock.

It sounds like Jan is specifically thinking that someone may (say) call 
domctl_lock(), then afterwards call domain_pause_except_self().

Of course, that would deadlock immediately, so would probably get caught before 
the patch even got to `git send-email`.  But it seems like a reasonable thing 
someone might want to do.

OTOH, I’m fine leaving making it recursive until someone discovers that it 
needs to be.

 -George

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