Re: [Xen-devel] [PATCHv6 5/5] gnttab: use per-VCPU maptrack free lists

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 23/04/15 17:11, Jan Beulich wrote:
>>>> On 22.04.15 at 18:00, <david.vrabel@xxxxxxxxxx> wrote:
>>  
>> +    v->maptrack_head = lgt->maptrack_pages * MAPTRACK_PER_PAGE;
>> +    if (v->maptrack_tail == MAPTRACK_TAIL)
>> +    {
>> +        v->maptrack_tail = (lgt->maptrack_pages * MAPTRACK_PER_PAGE)
>> +            + MAPTRACK_PER_PAGE - 1;
>> +        new_mt[i - 1].ref = MAPTRACK_TAIL;
>>      }
>>  
>> +    spin_lock(&lgt->maptrack_lock);
>> +    lgt->maptrack[lgt->maptrack_pages++] = new_mt;
>>      spin_unlock(&lgt->maptrack_lock);
> 
> The uses of ->maptrack_pages ahead of taking the lock can race
> with updates inside the lock. And with locking elsewhere dropped
> by the previous patch it looks like you can't update ->maptrack[]
> like you do (you'd need a barrier between the pointer store and
> the increment, and with that I think the lock would become
> superfluous if it was needed only for this update).

This was my mistake.  Malcolm's original patch had correct locking here.

> Also note the coding style issues in the changes above^(and also
> in code further down).
> 
>> -    return handle;
>> +    v->maptrack_limit = new_mt_limit;
>> +
>> +    return __get_maptrack_handle(lgt, v);
> 
> With the lock dropped, nothing guarantees this to succeed, which it
> ought to unless the table size reached its allowed maximum.

We've just added a new page of handles for this VCPU.  This will succeed.

David

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