[PATCH 2/2] rcu/tree: use __GFP_NO_LOCKS flag

Uladzislau Rezki (Sony)
2020-08-09T20:44:14Z

Enter the page allocator with newly introduced __GFP_NO_LOCKS flag
instead of former GFP_NOWAIT | __GFP_NOWARN sequence. Such approach
address two concerns. See them below:

a) If built with CONFIG_PROVE_RAW_LOCK_NESTING, the lockdep complains
about violation("BUG: Invalid wait context") of the nesting rules. It
does the raw_spinlock vs. spinlock nesting checks, i.e. it is not legal
to acquire a spinlock_t while holding a raw_spinlock_t.

Internally the kfree_rcu() uses raw_spinlock_t whereas the page allocator
internally deals with spinlock_t to access to its zones. The code also
can be broken from higher level of view:
<snip>
raw_spin_lock(&some_lock);
kfree_rcu(some_pointer, some_field_offset);
<snip>

b) If built with CONFIG_PREEMPT_RT. Please note, in that case spinlock_t
is converted into sleepable variant. Invoking the page allocator from
atomic contexts leads to: "BUG: scheduling while atomic".

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
kernel/rcu/tree.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 30e7e252b9e7..48cb64800108 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3327,7 +3327,7 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
* pages are available.
*/
bnode = (struct kvfree_rcu_bulk_data *)
- __get_free_page(GFP_NOWAIT | __GFP_NOWARN);
+ __get_free_page(__GFP_NO_LOCKS);
}

/* Switch to emergency path. */
--
2.20.1