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

[Rasmus Villemoes <ravi@prevas.dk>]

On Mon, Mar 03 2025, Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Mon, 3 Mar 2025 at 10:28, Oleg Nesterov <oleg@redhat.com> wrote:
>>
>> Stupid question... but do we really need to change the code which update
>> tail/head if we pack them into a single word?
>
> No. It's only the READ_ONCE() parts that need changing.
>
> See this suggested patch, which does something very similar to what
> you were thinking of.
>
> +/*
> + * 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/

Rasmus