Messages in this thread |  | | From | David Howells <> | Subject | Re: Why add the general notification queue and its sources | Date | Fri, 06 Sep 2019 11:09:17 +0100 |
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Linus Torvalds <torvalds@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
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