Re: PREEMPT_RT vs bcache

[Christoph Hellwig <hch@infradead.org>]

On Thu, Aug 08, 2013 at 12:43:26AM -0700, Kent Overstreet wrote:
> I seem to recall from looking at the logs that you just removed them
> because all the old users could be and were converted to something
> saner, for what they were doing (using them as completions, I want to
> say?)

We explicitly converted them away so that we could kill it.  This was
a joint project with Thomas.

> Bcache isn't using the rw sem as a completion though, it really is a
> read/write lock that protects a specific data structure, and  where
> we're taking a read lock for the duration of write IOs - and since bios
> are asynchronous, that's why we need the non_owner() bit.

Part of this commit was to make the rw_semaphore behaviour similar to
plain mutex, that is making sure there is exactly one owner and not
different processes locking/unlocking it.  This is useful for PI (that's
why the rt folks care), lock debugging and kinds of other use cases.