Re: [patch/rfc] eventfd semaphore-like behavior

[Michael Kerrisk <mtk.manpages-gM/Ye1E23mwN+BqQ9rBEUg@public.gmane.org>]

On Thu, Feb 5, 2009 at 12:27 PM, Davide Libenzi <davidel-AhlLAIvw+VEjIGhXcJzhZg@public.gmane.org> wrote:
> On Wed, 4 Feb 2009, Andrew Morton wrote:
>
>> On Wed, 4 Feb 2009 15:18:43 -0800 (PST)
>> Davide Libenzi <davidel-AhlLAIvw+VEjIGhXcJzhZg@public.gmane.org> wrote:
>>
>> > > > Simple test here:
>> > > >
>> > > > http://www.xmailserver.org/eventfd-sem.c
>> > > >
>> > > >
>> > > > Signed-off-by: Davide Libenzi <davidel-AhlLAIvw+VEjIGhXcJzhZg@public.gmane.org>
>> > > >
>> > > > +/*
>> > > > + * CAREFUL: Check include/asm-generic/fcntl.h when defining
>> > > > + * new flags, since they might collide with O_* ones. We want
>> > > > + * to re-use O_* flags that couldn't possibly have a meaning
>> > > > + * from eventfd, in order to leave a free define-space for
>> > > > + * shared O_* flags.
>> > > > + */
>> > > > +#define EFD_SEMAPHORE (1 << 0)
>> > > >  #define EFD_CLOEXEC O_CLOEXEC
>> > > >  #define EFD_NONBLOCK O_NONBLOCK
>> > > >
>> > > > +#define EFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK)
>> > > > +#define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE)
>> > >
>> > > How would you recommend that userspace determine whether its kernel
>> > > supports this feature, bearing in mind that someone might backport this
>> > > patch into arbitrarily earlier kernel versions?
>> > >
>> > > What should be userspace's fallback strategy if that support is not
>> > > present?
>> >
>> > #ifdef EFD_SEMAPHORE, maybe?
>>
>> That's compile-time.  People who ship binaries will probably want
>> to find a runtime thing for back-compatibility.
>
> I dunno. How do they actually do when we add new flags, like the O_ ones?

Maybe I missed something, but I think we're okay, aren't we?  Viz:

a) The glibc eventfd() wrapper invokes sys_eventfd2() (which allows a
flags arg) if it is available, and otherwise fall back to
sys_eventfd() (which does not support a flags arg).

b) If glibc falls back to sys_eventfd(), then it knows to reject
non-zero flags.  (The glibc wrapper already does this.)

c) If the old sys_eventfd2() is given a flag that it doesn't
recognize, then it fails with EINVAL.  (That check is already in the
code.)

So, userspace can determine whether EFD_SEMAPHORE is supported by not
getting an EINVAL error.  (Okay, this falls down for binaries that
bypass glibc's eventfd() wrapper, but there are unlikely to be such
binaries.)

Cheers,

Michael
-- 
Michael Kerrisk
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