Re: [PATCH RFC] big reader semaphore take#2

[Tejun Heo <htejun@gmail.com>]

Nick Piggin wrote:
> Tejun Heo wrote:
> 
>> Tejun Heo wrote:
>>
>>>  Hello guys,
>>>
>>> This is the second take of brsem (big reader semaphore).
>>>
>>> Nick, unfortunately, simple array of rwsem's does not work as lock
>>> holders are not pinned down to cpus and may release locks on other
>>> cpus.
>>>
> 
> [...]
> 
>>
>> (Nick, what do you think about the new implementation?)
>>
> 
> As I said, I think I'd prefer to see an implementation that returns
> a token from down_read to be used in up_read (ie. the slot # of the
> counter which has been downed).
> 

Oh... I read your response but thought that was only response to 'that 
wouldn't work' part.

> This obviously no longer makes it a drop in replacement for an rwsem.
> But could such a beast ever be considered so? Would that make your
> VFS patches really ugly?

I think Al Viro is on that now.

> The upshot of that would be that you could build the whole thing
> from rwsem infrastructure and have basically zero other locking
> mechanisms or complexity that you don't want in a synchronisation
> primitive.
> 

To certain extent, I do agree with you - it's safer/simpler..., but on 
the other hand, new brsem isn't that more complex and would perform 
almost identically without extra semantical baggage.  So, I thought it 
might be worth a bit more effort.

Hmm... So, array of rwsem's, it should be.

Thanks.

-- 
tejun