[RFC PATCH 04/14] pipe: Add O_NOTIFICATION_PIPE [ver #2]
David Howells
dhowells at redhat.com
Fri Nov 8 06:42:50 UTC 2019
Andy Lutomirski <luto at kernel.org> wrote:
> I can open a normal pipe from userspace (with pipe() or pipe2()), and
> I can have two threads. One thread writes to the pipe with write().
> The other thread writes with splice(). Everything works fine.
Yes. Every operation you do on a pipe from userspace is serialised with the
pipe mutex - and both ends share the same pipe.
> What's special about notifications?
The post_notification() cannot take the pipe mutex. It has to be callable
from softirq context. Linus's idea is that when you're actually altering the
ring pointers you should hold the wake-queue spinlock, and post_notification()
holds the wake queue spinlock for the duration of the operation.
This means that post_notification() can be writing to the pipe whilst a
userspace-invoked operation is holding the pipe mutex and is also doing
something to the ring.
David
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