Re: [RFC PATCH] x86 alternatives : fix LOCK_PREFIX race with preemptible kernel and CPU hotplug

[Jeremy Fitzhardinge <jeremy@goop.org>]

Mathieu Desnoyers wrote:
> * Jeremy Fitzhardinge (jeremy@goop.org) wrote:
>   
>> Mathieu Desnoyers wrote:
>>     
>>> * H. Peter Anvin (hpa@zytor.com) wrote:
>>>   
>>>       
>>>> Mathieu Desnoyers wrote:
>>>>     
>>>>         
>>>>> I can't argue about the benefit of using VM CPU pinning to manage
>>>>> resources because I don't use it myself, but I ran some tests out of
>>>>> curiosity to find if uncontended locks were that cheap, and it turns out
>>>>> they aren't. Here are the results :
>>>>> Xeon 2.0GHz
>>>>> Summary
>>>>> make -j1 kernel/      33.94 +/- 0.07         34.91 +/- 0.27      2.8 %
>>>>> hackbench 50           2.99 +/- 0.01          3.74 +/- 0.01     25.1 %
>>>>> 1 CPU, replace smp lock prefixes with DS segment selector prefixes
>>>>> 1 CPU, noreplace-smp
>>>>>       
>>>>>           
>>>> For reference, could you also compare replace smp lock with NOPs?
>>>>
>>>> 	-hpa
>>>>     
>>>>         
>>> Sure, here are the updated tables. Basically, they show no significant
>>> difference between the NOP and the DS segment selector prefix
>>> approaches.
>>>   
>>>       
>> BTW, are you changing the initial prefix to DS too?  Ie, are you doing a 
>> nop->lock->ds transition, or ds->lock->ds?
>>
>>    J
>>     
>
> Yeah, I thought about this case yesterday, good thing you ask.
>
> include/asm-x86/alternative.h defines LOCK_PREFIX as :
>
> #define LOCK_PREFIX \
>                 ".section .smp_locks,\"a\"\n"   \
>                 _ASM_ALIGN "\n"                 \
>                 _ASM_PTR "661f\n" /* address */ \
>                 ".previous\n"                   \
>                 "661:\n\tlock; "
>
> So we have the locked instructions built into the kernel, not the nop'd
> one. Therefore, the only transition I am doing for my benchmarks is :
>
> lock->ds
>
> but I tried to switch back to SMP and it worked fine.
>   

Ah, OK.  I'd thought we started unlocked, but given that I've just been 
disassembling the kernel and looking at the lock prefixes, that's a bit 
of a braino on my part.

BTW, using the ds prefix allows us to undo the hack of dealing with 
locked instructions with exception handlers.  There was a bug where if 
we do lock->nop, then the address of a faulting instruction changes, so 
we need exception records for both the locked and unlocked forms.  Using 
ds means the instruction size doesn't change, so we only need one 
exception record.  I don't remember off hand where that happens.

    J