diff options
author | Milton Miller <miltonm@bga.com> | 2011-05-24 20:34:18 +0000 |
---|---|---|
committer | Benjamin Herrenschmidt <benh@kernel.crashing.org> | 2011-05-26 13:38:59 +1000 |
commit | 9b7882515864117d0015a3484c0ba0eee6713de9 (patch) | |
tree | 727b76078328927ba87f060b7bb57219ead3f314 /arch | |
parent | 2e455257d143f54b44701e947a092d513889d01c (diff) |
powerpc/irq: Protect irq_radix_revmap_lookup against irq_free_virt
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.
Signed-off-by: Milton Miller <miltonm@bga.com>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Diffstat (limited to 'arch')
-rw-r--r-- | arch/powerpc/kernel/irq.c | 7 |
1 files changed, 5 insertions, 2 deletions
diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c index ac4d29119f3..6cb3fcd7fc3 100644 --- a/arch/powerpc/kernel/irq.c +++ b/arch/powerpc/kernel/irq.c @@ -893,10 +893,13 @@ unsigned int irq_radix_revmap_lookup(struct irq_host *host, 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. |