Re: [PATCH 2/3] rwsem: Implement down_read_interruptible

From: Waiman Long
Date: Tue Dec 08 2020 - 10:36:43 EST


On 12/8/20 4:12 AM, David Laight wrote:
From: Waiman Long
Sent: 07 December 2020 19:02
...
How much more difficult would it be to also add a timeout option?
I looked at adding one to the mutex code - and fell into a big pile
of replicated code.

ISTM that one the initial locked exchange (and spin) fails a few
extra instructions when heading for the sleep don't really matter

Actually, I had tried that before. See

https://lore.kernel.org/lkml/20190911150537.19527-1-longman@xxxxxxxxxx/

That is for rwsem, but the same can be done for mutex. However, Peter
didn't seem to like the idea of a timeout parameter. Anyway, it is
certainly doable if there is a good use case for it.
'Unfortunately' my use-case if for an out-of-tree driver.

The problem I was solving is a status call blocking because
some other code is 'stuck' (probably an oops) with a mutex held.

The code used to use down_timeout() (it was written for 2.4).
When I changed to mutex_(to get optimistic spinning) I lost
the ability to do the timeouts.

The primary reason for sending out that patchset was to work around some circular locking problem in existing code even though these circular locking scenarios are not likely to happen. Your case is certainly another potential circular locking problem as well.

Cheers,
Longman