lkml.org 
[lkml]   [2003]   [Sep]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH 2/2] Futex non-page-pinning fix
Date
In message <20030902065144.GC7619@mail.jlokier.co.uk> 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.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2005-03-22 13:48    [from the cache]
©2003-2011 Jasper Spaans