Messages in this thread | | | Subject | Re: Change in functionality of futex() system call. | From | Eric Dumazet <> | Date | Tue, 07 Jun 2011 05:49:22 +0200 |
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Le lundi 06 juin 2011 à 20:13 -0700, Darren Hart a écrit : > > 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. > > Does anyone with a longer history working with futexes than I have an > opinion on this? Is support for RO private mappings part of our futex > API, or was it an unintentional side effect of the futex simply being a > userspace address. >
I personnally dont care as I dont use ro mappings for my futexes land, but I can feel the pain of people discovering yet another incompatibility in their user apps after a kernel upgrade, spending so much time to find the root of the problem (hey, not everybody is a kernel hacker)
If we think about it, futex_wait() should not touch memory, only read it. Some smart layer could be upset by this (valgrind ?)
Its like saying write(int fd, const void *buffer, size_t count) could try to do a COW on buffer, because it makes kernel programmer life more comfortable, this makes litle sense to me IMHO.
Part of the problem comes from futex() syscall being a multiplexor. What a mess.
If we had a clean API at the beginning, then we would have :
int sys_futex_wait(const void *futex, int val, const struct timespec *t);
And really, doing COW in futex_wait() would clearly be wrong.
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