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SubjectRe: [PATCH RFC] big reader semaphore take#2
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.


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