Messages in this thread | | | Date | Tue, 07 Jun 2011 10:44:20 -0400 | From | Andy Lutomirski <> | Subject | Re: Change in functionality of futex() system call. |
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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.
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