Why add the general notification queue and its sources

[David Howells]

Linus Torvalds <torvalds at linux-foundation.org> wrote:

> But it's *literally* just finding the places that work with
> pipe->curbuf/nrbufs and making them use atomic updates.

No.  It really isn't.  That's two variables that describe the occupied section
of the buffer.  Unless you have something like a 68020 with CAS2, or put them
next to each other so you can use CMPXCHG8, you can't do that.

They need converting to head/tail pointers first.

> They really would work with almost anything. You could even mix-and-match
> "data generated by kernel" and "data done by 'write()' or 'splice()' by a
> user process".

Imagine that userspace writes a large message and takes the mutex.  At the
same time something in softirq context decides *it* wants to write a message -
it can't take the mutex and it can't wait, so the userspace write would have
to cause the kernel message to be dropped.

What I would have to do is make a write to a notification pipe go through
post_notification() and limit the size to the maximum for a single message.

Much easier to simply suppress writes and splices on pipes that have been set
up to be notification queues - at least for now.

David