Re: Locking in disk(9) api

[Thor Lancelot Simon <tls%panix.com@localhost>]

On Tue, Dec 29, 2009 at 03:53:58PM -0800, John Nemeth wrote:
> On May 21,  1:16pm, Thor Lancelot Simon wrote:
> } On Wed, Dec 30, 2009 at 12:32:02AM +0100, Michael van Elst wrote:
> } > 
> } > Well, originally this was read via kvm and always in a state of flux.
> } 
> } I think this is almost the canonical example of data we should not lock.
> } The cost of using locking *or* atomic operations for statistics update
> } will be extreme on busy MP systems, and there's almost no real benefit.
> 
>      In the case in question the real benefit is avoiding a panic when
> dk_stats->io_busy goes negative.

If the system can panic because a "statistic" goes negative, that's
a plenty serious bug all its own.

If we can't figure out how to choose appropriate size and signedness
for statistics so the kernel can update them unlocked with, at worst,
a risk that readers will see a slightly outdated value, we're a lot
dumber than I think we are.

Thor