Re: [Qemu-devel] [patch v4 12/16] e1000: apply fine lock on e1000

[Jan Kiszka <jan.kiszka@web.de>]

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On 2012-10-25 11:01, Avi Kivity wrote:
> On 10/24/2012 09:17 AM, Jan Kiszka wrote:
>>>>
>>>> This is ugly for many reasons. First of all, it is racy as the register
>>>> content may change while dropping the device lock, no? Then you would
>>>> raise or clear an IRQ spuriously.
>>>>
>>> Device state's intact is protected by busy flag, and will not broken
>>
>> Except that the busy flag concept is broken in itself.
> 
> How do we fix an mmio that ends up mmio'ing back to itself, perhaps
> indirectly?  Note this is broken in mainline too, but in a different way.
> 
> Do we introduce clever locks that can detect deadlocks?

That problem is already addressed (to my understanding) by blocking
nested MMIO in general. The brokenness of the busy flag is that it
prevents concurrent MMIO by dropping requests.

> 
>> I see that we have a all-or-nothing problem here: to address this
>> properly, we need to convert the IRQ path to lock-less (or at least
>> compatible with holding per-device locks) as well.
> 
> There is a transitional path where writing to a register that can cause
> IRQ changes takes both the big lock and the local lock.
> 
> Eventually, though, of course all inner subsystems must be threaded for
> this work to have value.
> 

But that transitional path must not introduce regressions. Opening a
race window between IRQ cause update and event injection is such a
thing, just like dropping concurrent requests on the floor.

Jan


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