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SubjectRe: [PATCH 9/9] qspinlock,x86,kvm: Implement KVM support for paravirt qspinlock
On 03/19/2015 06:01 AM, Peter Zijlstra wrote:
> On Wed, Mar 18, 2015 at 10:45:55PM -0400, Waiman Long wrote:
>> On 03/16/2015 09:16 AM, Peter Zijlstra wrote:
>> I do have some concern about this call site patching mechanism as the
>> modification is not atomic. The spin_unlock() calls are in many places in
>> the kernel. There is a possibility that a thread is calling a certain
>> spin_unlock call site while it is being patched by another one with the
>> alternative() function call.
>>
>> So far, I don't see any problem with bare metal where paravirt_patch_insns()
>> is used to patch it to the move instruction. However, in a virtual guest
>> enivornment where paravirt_patch_call() was used, there were situations
>> where the system panic because of page fault on some invalid memory in the
>> kthread. If you look at the paravirt_patch_call(), you will see:
>>
>> :
>> b->opcode = 0xe8; /* call */
>> b->delta = delta;
>>
>> If another CPU reads the instruction at the call site at the right moment,
>> it will get the modified call instruction, but not the new delta value. It
>> will then jump to a random location. I believe that was causing the system
>> panic that I saw.
>>
>> So I think it is kind of risky to use it here unless we can guarantee that
>> call site patching is atomic wrt other CPUs.
> Just look at where the patching is done:
>
> init/main.c:start_kernel()
> check_bugs()
> alternative_instructions()
> apply_paravirt()
>
> We're UP and not holding any locks, disable IRQs (see text_poke_early())
> and have NMIs 'disabled'.

You are probably right. The initial apply_paravirt() was done before the
SMP boot. Subsequent ones were at kernel module load time. I put a
counter in the __native_queue_spin_unlock() and it registered 26949
unlock calls in a 16-cpu guest before it got patched out.

The panic that I observed before might be due to some coding error of my
own.

-Longman


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