Hi Peter, On Fri, 25 Jul 2008 09:49:37 +0200 Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > On Thu, 2008-07-24 at 14:18 +0200, Sebastien Dugue wrote: > > On Thu, 24 Jul 2008 21:11:34 +1000 Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote: > > > > > On Thursday 24 July 2008 20:50, Sebastien Dugue wrote: > > > > From: Sebastien Dugue <sebastien.dugue@xxxxxxxx> > > > > Date: Tue, 22 Jul 2008 11:56:41 +0200 > > > > Subject: [PATCH][RT] powerpc - Make the irq reverse mapping radix tree > > > > lockless > > > > > > > > The radix tree used by interrupt controllers for their irq reverse > > > > mapping (currently only the XICS found on pSeries) have a complex locking > > > > scheme dating back to before the advent of the concurrent radix tree on > > > > preempt-rt. > > > > > > > > Take advantage of this and of the fact that the items of the tree are > > > > pointers to a static array (irq_map) elements which can never go under us > > > > to simplify the locking. > > > > > > > > Concurrency between readers and writers are handled by the intrinsic > > > > properties of the concurrent radix tree. Concurrency between the tree > > > > initialization which is done asynchronously with readers and writers access > > > > is handled via an atomic variable (revmap_trees_allocated) set when the > > > > tree has been initialized and checked before any reader or writer access > > > > just like we used to check for tree.gfp_mask != 0 before. > > > > > > Hmm, RCU radix tree is in mainline too for quite a while. I thought > > > Ben had already converted this code over ages ago... > > > > Mainline does not have the concurrent radix tree which this patch > > is based on, but maybe it's overkill and the RCU radix tree is enough. > > Not sure, will have to think about it a bit more. > > Should be. The model of the concurrent radix tree can be mapped to > spinlock + rcu radix tree. > > So instead of: > > > + DEFINE_RADIX_TREE_CONTEXT(ctx, tree); > > + radix_tree_lock(&ctx); > > + radix_tree_insert(ctx.tree, hwirq, &irq_map[virq]); > > + radix_tree_unlock(&ctx); > > > you then write: > > spin_lock(&host->revmap_data.tree_lock); > radix_tree_insert(&host->revmap_data.tree, hwirq, &irq_map[virq]); > spin_unlock(&host->revmap_data.tree_lock); > Cool, that will indeed makes it much easier to have something applicable to mainline which works with preempt-rt. > > The only advantage of the concurrent radix tree over this model is that > it can potentially do multiple modification operations at the same time. Well in theory that can happen if a module is loaded which creates a mapping while another one is unloaded at the same time. The time window is pretty narrow, but still present nonetheless. That's why I chose to use the concurrent version. > > Still, cool that you used it ;-) Yep, looked like what was needed until I realized it was not available in mainline. Nice work though and good paper for explaining it all. Sebastien. -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
