Messages in this thread | | | From | Andrew Lutomirski <> | Date | Tue, 7 Jun 2011 14:43:06 -0400 | Subject | Re: Change in functionality of futex() system call. |
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On Tue, Jun 7, 2011 at 11:58 AM, Eric Dumazet <eric.dumazet@gmail.com> wrote: > Le mardi 07 juin 2011 à 10:44 -0400, Andy Lutomirski a écrit : >> On 06/06/2011 11:13 PM, Darren Hart wrote: >> > >> > >> > On 06/06/2011 11:11 AM, Eric Dumazet wrote: >> >> Le lundi 06 juin 2011 à 10:53 -0700, Darren Hart a écrit : >> >>> >> >> >> >>> If I understand the problem correctly, RO private mapping really doesn't >> >>> make any sense and we should probably explicitly not support it, while >> >>> special casing the RO shared mapping in support of David's scenario. >> >>> >> >> >> >> We supported them in 2.6.18 kernels, apparently. This might sounds >> >> stupid but who knows ? >> > >> > >> > I guess this is actually the key point we need to agree on to provide a >> > solution. This particular case "worked" in 2.6.18 kernels, but that >> > doesn't necessarily mean it was supported, or even intentional. >> > >> > It sounds to me that we agree that we should support RO shared mappings. >> > The question remains about whether we should introduce deliberate >> > support of RO private mappings, and if so, if the forced COW approach is >> > appropriate or not. >> > >> >> I disagree. >> >> FUTEX_WAIT has side-effects. Specifically, it eats one wakeup sent by >> FUTEX_WAKE. So if something uses futexes on a file mapping, then a >> process with only read access could (if the semantics were changed) DoS >> the other processes by spawning a bunch of threads and FUTEX_WAITing >> from each of them. >> >> If there were a FUTEX_WAIT_NOCONSUME that did not consume a wakeup and >> worked on RO mappings, I would drop my objection. > > If a group of cooperating processes uses a memory segment to exchange > critical information, do you really think this memory segment will be > readable by other unrelated processes on the machine ?
Depends on the design.
I have some software I'm working on that uses shared files and could easily use futexes. I don't want random read-only processes to interfere with the futex protocol.
> > How is this related to futex code ?
Because this usage is currently safe and would become unsafe with the proposed change.
> > Same problem for legacy IPC (shm, msg, sem) : Appropriate protections > are needed, obviously. > > BTW, kernel/futex.c uses a global hash table (futex_queues[256]) and a > very predictable hash_futex(), so its easy to slow down futex users...
There's a difference between slowing down users by abusing a kernel hash and deadlocking users by eating a wakeup. (If you eat a wakeup the wakeup won't magically come back later. It's gone.)
--Andy -- 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/
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