[Mesa-dev] [PATCH] os: add spinlocks

[Keith Whitwell]

On Wed, 2010-12-15 at 09:19 -0800, Kristian Høgsberg wrote:
> On Wed, Dec 15, 2010 at 10:10 AM, Thomas Hellstrom
> <thellstrom at vmware.com> wrote:
> ...
> > Given this, I would advise strongly against building spinlocks into any code
> > that might be run on a uni-processor system.  Particularly gallium utility
> > code.
> > If we want to get rid of unnecessary locking overhead we should probably fix
> > the code up to avoid taking the locks when not strictly needed.
> 
> Another option is to rethink/refactor the code in question to just
> take the locks less.  Use per thread (context) state instead where
> possible and batch updates to global state so you can take the lock
> and do a bunch of stuff.  For example, if you're putting many items
> back on a global free list, just put them back on a local free list
> one by one, and then take the lock and then merge the thread local
> free list into the global list (should be a constant time operation)
> eventually.
> 
> I know it's easier said than done, but if locking is showing up on the
> profile, I think "use less locking" is a better fix than  "use faster
> locking".

I think one thing that's going on here is we've made it too easy to
share objects between threads/contexts, or perhaps too hard to create
context-private objects.

Before making more guesses though I'd be interested to get more
information about what these locks are protecting & what the
circumstances are.

Keith