Re: [PATCH] Make futex waiters take an mm or inode reference

[Rusty Russell]

In message <Pine.LNX.4.44.0309081144390.3202-100000@xxxxxxxxxxxxx> you write:
> So is there any reason to really having "private.mm" AT ALL? From what I
> can tell, it is not actually ever used (all "mm" users are "current->mm"),
> so I don't see the point of incrementing a count for it either.
> 
> Or did I miss something?

Yes.  Firstly, you can't do "wake one" if the one you wake might be
some completely unrelated process which happens to use the same
address.  Secondly, I implemented fair futexes by relying on the
return value of FUTEX_WAKE to indicate how many people were woken: you
set the futex to a magic value, call FUTEX_WAKE(1), and if it returns
1, you've "passed" the futex directly, otherwise you unlock the futex
like normal.  This is surprisingly useful for implementing
"drop_futex_if_someone_is_waiting()" in cleanup threads etc.

Sorry,
Rusty.
--
  Anyone who quotes me in their sig is an idiot. -- Rusty Russell.
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