Messages in this thread | | | Date | Tue, 07 Jun 2011 08:56:34 -0700 | From | Darren Hart <> | Subject | Re: Change in functionality of futex() system call. |
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On 06/07/2011 07:44 AM, Andy Lutomirski wrote: > 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.
This sounds like an argument for properly managing file permissions and carefully selecting the mapping backing your futex word - but I don't see this as compelling rationale to disable RO support entirely and certainly not to add yet another futex op code.
-- Darren Hart Intel Open Source Technology Center Yocto Project - Linux Kernel -- 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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