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

[Rusty Russell]

In message <20030902065144.GC7619@xxxxxxxxxxxxxxxxxx> you write:
> What happens after this sequence:
> 
> 	1. process A forks, making process B
> 	2. B does FUTEX_FD, or splits into threads and one does FUTEX_WAIT,
> 	   on a private page that has not been written to since the fork
> 	3. A does FUTEX_WAIT on the same address
> 	3. The page is swapped out
> 	4. B does FUTEX_WAKE at the same address
> 
> Won't the futex be hashed on the swap entry at step 4, so that
> both processes are woken, yet only the waiter in B should be woken?

Part of step (4) is to swap the page back in (see __pin_page).

> Related: could COW sharing after fork() explain the spurious wakeups I
> saw mentioned earlier in the thread?

In case others are sharing this misconception: there *are* no spurious
wakeups.  But if they were to happen, the current code doesn't handle
them correctly, unlike every other primitive I know of in the kernel,
which is why I fixed it while tidying the code.

I don't know of a rule which says "thou shalt not wake a random thread
in the kernel": for all I know wierd things like CPU hotplug or
software suspend may do this in the future.

Hope that clarifies,
Rusty.
--
  Anyone who quotes me in their sig is an idiot. -- Rusty Russell.
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