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

[Oleg Nesterov]

On 03/03, Linus Torvalds wrote:
>
> There's currently a fair number of open-coded assignments:
>
>     git grep -E 'pipe->((tail)|(head)).*=' fs/
>
> and some of those are under specific locking rules together with other
> updates (ie the watch-queue 'note_loss' thing.

Stupid question... but do we really need to change the code which update
tail/head if we pack them into a single word?

I mean,

	-	unsigned int head;
	-	unsigned int tail;
	+	union {
	+		struct {
	+			u16 head, tail;
	+		}
	+
	+		__u32   head_tail;
	+	}

Now pipe_writebale() can read do READ_ONCE(pipe->head_tail) "atomically"
without preemption and this is all we need, no?

Yes, pipe_writable() should take endianess into account, but this is
simple...

Oleg