[PATCH rcu 5/8] rcu/kvfree: Use consistent krcp when growing kfree_rcu() page cache

From: Paul E. McKenney
Date: Wed May 10 2023 - 13:03:20 EST

From: Zqiang <qiang1.zhang@xxxxxxxxx>

The add_ptr_to_bulk_krc_lock() function is invoked to allocate a new
kfree_rcu() page, also known as a kvfree_rcu_bulk_data structure.
The kfree_rcu_cpu structure's lock is used to protect this operation,
except that this lock must be momentarily dropped when allocating memory.
It is clearly important that the lock that is reacquired be the same
lock that was acquired initially via krc_this_cpu_lock().

Unfortunately, this same krc_this_cpu_lock() function is used to
re-acquire this lock, and if the task migrated to some other CPU during
the memory allocation, this will result in the kvfree_rcu_bulk_data
structure being added to the wrong CPU's kfree_rcu_cpu structure.

This commit therefore replaces that second call to krc_this_cpu_lock()
with raw_spin_lock_irqsave() in order to explicitly acquire the lock on
the correct kfree_rcu_cpu structure, thus keeping things straight even
when the task migrates.

Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
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 426f1f3bb5f2..51d84eabf645 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3279,7 +3279,7 @@ add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp,
// scenarios.
bnode = (struct kvfree_rcu_bulk_data *)
- *krcp = krc_this_cpu_lock(flags);
+ raw_spin_lock_irqsave(&(*krcp)->lock, *flags);

if (!bnode)