On Wed, May 25, 2011 at 01:34:18AM -0500, Milton Miller wrote: > The radix-tree code uses call_rcu when freeing internal elements. > We must protect against the elements being freed while we traverse > the tree, even if the returned pointer will still be valid. > > While preparing a patch to expand the context in which > irq_radix_revmap_lookup will be called, I realized that the > radix tree was not locked. > > When asked > > For a normal call_rcu usage, is it allowed to read the structure in > irq_enter / irq_exit, without additional rcu_read_lock? Could an > element freed with call_rcu advance with the cpu still between > irq_enter/irq_exit (and irq_disabled())? > > Paul McKenney replied: > > Absolutely illegal to do so. OK for call_rcu_sched(), but a > flaming bug for call_rcu(). > > And thank you very much for finding this!!! > > Further analysis: > > In the current CONFIG_TREE_RCU implementation. CONFIG_TREE_PREEMPT_RCU > (and CONFIG_TINY_PREEMPT_RCU) uses explicit counters. > > These counters are reflected from per-CPU to global in the > scheduling-clock-interrupt handler, so disabling irq does prevent the > grace period from completing. But there are real-time implementations > (such as the one use by the Concurrent guys) where disabling irq > does -not- prevent the grace period from completing. > > > While an alternative fix would be to switch radix-tree to rcu_sched, I > don't want to audit the other users of radix trees (nor put alternative > freeing in the library). The normal overhead for rcu_read_lock and > unlock are a local counter increment and decrement. > > This does not show up in the rcu lockdep because in 2.6.34 commit > 2676a58c98 (radix-tree: Disable RCU lockdep checking in radix tree) > deemed it too hard to pass the condition of the protecting lock > to the library.
Reviewed-by: Paul E. McKenney <paul...@linux.vnet.ibm.com> > Signed-off-by: Milton Miller <milt...@bga.com> > > Index: work.git/arch/powerpc/kernel/irq.c > =================================================================== > --- work.git.orig/arch/powerpc/kernel/irq.c 2011-05-24 21:14:30.860096118 > -0500 > +++ work.git/arch/powerpc/kernel/irq.c 2011-05-24 21:15:55.350096024 > -0500 > @@ -893,10 +893,13 @@ unsigned int irq_radix_revmap_lookup(str > return irq_find_mapping(host, hwirq); > > /* > - * No rcu_read_lock(ing) needed, the ptr returned can't go under us > - * as it's referencing an entry in the static irq_map table. > + * The ptr returned references the static global irq_map. > + * but freeing an irq can delete nodes along the path to > + * do the lookup via call_rcu. > */ > + rcu_read_lock(); > ptr = radix_tree_lookup(&host->revmap_data.tree, hwirq); > + rcu_read_unlock(); > > /* > * If found in radix tree, then fine. _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
