Unexpected scheduling with mutexes

[Valdis Kl=?utf-8?Q?=c4=93?=tnieks]

On Fri, 29 Mar 2019 21:01:58 +0100, Greg KH said:

> But if you are trying to somehow create a real api that you have to
> enforce the passing off of writing data from two different character
> devices in an interleaved format, you are doing this totally wrong, as
> this is not going to work with a simple mutex, as you have found out.

There's almost always an even more fundamental issue here - I've seen plenty of
people attempt to do this sort of thing.  But invariably, they have little to
no explanation of what semantics they think are correct. I'm not sure who are
crazier - the people who try to do kernel-side locking for "exclusive" use of a
device, or the people who don't understand why having 3 different programs
trying to talk to /dev/ttyS0 at once will only lead to tearns and anguish...

(Though recently, I discovered that there are no bad ideas so obvious that
somebody won't try to re-invent them.  I caught a software package that *really*
should know better using "does DBus have an entry for this object?" as a lock.)

> Try to take USB out of the picture as well as userspace, and try running
> two kernel threads trying to grab a mutex and then print out "A" or "B"
> to the kernel log and then give it up.  Is that output nicely
> interleaved or is there some duplicated messages.[1]

> [1] Extra bonus points for those that recognize this task...

Been there, done that, got the tire marks to prove it. :)