Re: [PATCH] pipe_read: don't wake up the writer if the pipe is still full

[Rasmus Villemoes <ravi@prevas.dk>]

On Wed, Mar 05 2025, Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Wed, 5 Mar 2025 at 05:31, Rasmus Villemoes <ravi@prevas.dk> wrote:
>>
>> On Mon, Mar 03 2025, Linus Torvalds <torvalds@linux-foundation.org> wrote:
>>
>> > +/*
>> > + * We have to declare this outside 'struct pipe_inode_info',
>> > + * but then we can't use 'union pipe_index' for an anonymous
>> > + * union, so we end up having to duplicate this declaration
>> > + * below. Annoying.
>> > + */
>> > +union pipe_index {
>> > +     unsigned long head_tail;
>> > +     struct {
>> > +             pipe_index_t head;
>> > +             pipe_index_t tail;
>> > +     };
>> > +};
>> > +
>>
>> -fms-extensions ? Willy wanted to add that for use in mm/ some years ago
>> [*], and it has come up a few other times as well.
>>
>> [*] https://lore.kernel.org/lkml/20180419152817.GD25406@bombadil.infradead.org/
>
> Oh, I was unaware of that extension, and yes, it would have been
> lovely here, avoiding that duplicate union declaration.
>
> But it does require clang support - I see that clang has a
> '-fms-extensions' as well, so it's presumably there.

Yes, it seems they do have it, but for mysterious reasons saying
-fms-extensions is not quite enough to convince clang that one does
intend to use that MS extension, one also has to say
-Wno-microsoft-anon-tag, or it complains

warning: anonymous unions are a Microsoft extension [-Wmicrosoft-anon-tag]

Also, the warning text is somewhat misleading; anon unions itself have
certainly been a gcc extension since forever, and nowadays a C11 thing,
and clang has a separate -Wpedantic warning for that when using
-std=c99:

warning: anonymous unions are a C11 extension [-Wc11-extensions]

The -W flag name actually suggests an improvement to the warning
"_tagged_ anonymous unions are a Microsoft extension", but I really
wonder why -fms-extensions isn't sufficient to silence that in the first
place. Also, the warning seems to be on by default; it's not some
-Wextra or -Wpedantic thing.

cc += Nick

Rasmus