linux-kernel - Re: [PATCH] seqlock: serialize against writers

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Date:	Fri, 29 Aug 2008 13:00:26 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Andi Kleen <andi@...stfloor.org>,
	Gregory Haskins <gregory.haskins@...il.com>, mingo@...e.hu,
	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	linux-rt-users@...r.kernel.org
Subject: Re: [PATCH] seqlock: serialize against writers

Steven Rostedt wrote:
> The subject forgot to add "RT" in the brackets.
>
> On Fri, 29 Aug 2008, Andi Kleen wrote:
>
>   
>>> I could just force all of the seqbegins to hit the slowpath by hacking
>>> the code and see what happens (aside from slowing down, of course ;)
>>>       
>> Only if you don't believe it will really crash? I think it's pretty
>> clear even without trying it.
>>
>>     
>>> Question: Which seqlock_t does userspace use?  I assume it uses
>>> seqlock_t and not raw_seqlock_t. 
>>>       
>>> But the only reason that I ask is that
>>> I converted raw_seqlock_t to use the new style as well to be consistent,
>>>       
>> There's no raw_seqlock_t anywhere in mainline?
>>     
>
> Nope, raw_seqlock_t in -rt is equivelant to seqlock_t in mainline.
>
>   
>> Anyways the variable is declared (in mainline) in asm-x86/vgtod.h 
>>
>>     
>>> even though it is not strictly necessary for the same reasons.  So if
>>> perchance userspace uses the raw variant, I could solve this issue by
>>> only re-working the seqlock_t variant.  Kind of a long shot, but figured
>>> I would mention it :)
>>>       
>> I guess you could define a new seqlock_t which is explicitely user space
>> safe. That might avoid such issues in the future. But then
>> that would likely require some code duplication and be ugly.
>>
>> On the other hand whatever problem you fixing in the kernel
>> (to be honest it's still unclear to me what the problem is)
>> needs to be likely fixed for the userland lock too.
>>     
>
> I'm not convinced that the raw_seqlocks (mainline normal seqlocks) has a 
> problem anyway.
>   
(continuing from IRC)

Agreed.   I converted them to be consistent.  Steve just told me that
userspace actually uses the raw_seqlock_t variant, so the answer is
simple.  Just leave raw_seqlock_t alone and the patch will work fine.

Thoughts?

-Greg



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