Re: Change in functionality of futex() system call.

[Eric Dumazet]

Le lundi 06 juin 2011 Ã 20:13 -0700, Darren Hart a Ãcrit :
> 
> On 06/06/2011 11:11 AM, Eric Dumazet wrote:
> > Le lundi 06 juin 2011 Ã 10:53 -0700, Darren Hart a Ãcrit :
> >>
> > 
> >> 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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