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

[Michael Kerrisk]

On Thu, Feb 5, 2009 at 12:55 PM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, 4 Feb 2009 15:27:45 -0800 (PST)
> Davide Libenzi <davidel@xxxxxxxxxxxxxxx> wrote:
>
>> On Wed, 4 Feb 2009, Andrew Morton wrote:
>>
>> > On Wed, 4 Feb 2009 15:18:43 -0800 (PST)
>> > Davide Libenzi <davidel@xxxxxxxxxxxxxxx> wrote:
>> >
>> > > > > Simple test here:
>> > > > >
>> > > > > http://www.xmailserver.org/eventfd-sem.c
>> > > > >
>> > > > >
>> > > > > Signed-off-by: Davide Libenzi <davidel@xxxxxxxxxxxxxxx>
>> > > > >
>> > > > > +/*
>> > > > > + * 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?
>>
>
> Dunno.  Probably try the syscall and see if it returned -EINVAL.  Does
> that work in this case?

As youll have seen by now, Ulrich and I noted that it works.

> If so, it would be sensible to mention this in
> the description somewhere as the approved probing method and to
> maintain it.

I'll add something to the man page, as this patch progresses.

Cheers,

Michael


-- 
Michael Kerrisk
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