Messages in this thread | | | Subject | Re: Change in functionality of futex() system call. | From | Eric Dumazet <> | Date | Tue, 07 Jun 2011 17:58:20 +0200 |
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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 ?
How is this related to futex code ?
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...
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