Re: [PATCH 2/2] Futex non-page-pinning fix

[Andrew Morton]

Muli Ben-Yehuda <mulix@xxxxxxxxx> wrote:
>
> > > umm, how about hashing only on offset into page?  That reduces the number of
>  > > threads which need to be visited in futex_wake() by a factor of up to 1024.
>  > 
>  > How likely do you consider it that we then get a major collision?
>  > I wouldn't be surprised if, say, glibc lays some hot futexes out in a
>  > way that's the same for all processes in the system, like start of the
>  > page.... Might as well not hash :)
> 
>  How about combining something that's shared to all of the threads that
>  share a futex but not system wide (the mm?) with something simple that
>  won't change, like the page offset?

The mm's could well be independent.

Some userspace help would be needed to avoid defeating the hash.  In the
case where a bunch of threads with a shared mm are waiting on the same
futex things should automatically be OK.

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