Re: [rfc] "fair" rw spinlocks

[Linus Torvalds <torvalds@linux-foundation.org>]

On Tue, 1 Dec 2009, Arnd Bergmann wrote:
> On Monday 30 November 2009, Linus Torvalds wrote:
> > The best option really would be to try to make it all use RCU, rather than 
> > paper over things. That really should improve performance.
> 
> Are there any writers at interrupt time?

No, there can't be. That would already be a deadlock, since we take the 
read lock without irq protection (exactly because many of the read-lockers 
are pretty performance-sensitive).

> If not, another option might be to first convert all the readers that 
> can happen from interrupts to RCU, which lets us get rid of the irq 
> disable in the write path.

If you convert the irq readers, you generally really need to convert the 
rest too. In particular, you still need to convert the write-side to use 
the RCU versions of the insert/remove code, and to free the things from 
RCU in order for it all to be safe (think: irq reader on another CPU than 
the writer, now without any locking).

So you really don't win all that much. At a minimum, you always have to 
convert all the writers to use RCU (even if you then keep the rwlock as 
the exclusion model), and since that involves a large portion of the 
complexity (including at least the RCU freeing side), what you end up with 
is that you can avoid converting _some_ of the readers.

So I do agree that you can do things in two stages, but I suspect the irq 
disable on the write path part is the least of our problems.

				Linus