doc: RCU callback locks need only _bh, not necessarily _irq
The UP.rst file calls for locks acquired within RCU callback functions
to use _irq variants (spin_lock_irqsave() or similar), which does work,
but can be overkill. This commit therefore instead calls for _bh variants
(spin_lock_bh() or similar), while noting that _irq does work.
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Signed-off-by: Jiunn Chang <c0d1n61at3@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/RCU/UP.rst b/Documentation/RCU/UP.rst
index 67715a47..e26dda2 100644
--- a/Documentation/RCU/UP.rst
+++ b/Documentation/RCU/UP.rst
@@ -113,12 +113,13 @@
Answer to Quick Quiz #2:
What locking restriction must RCU callbacks respect?
- Any lock that is acquired within an RCU callback must be
- acquired elsewhere using an _irq variant of the spinlock
- primitive. For example, if "mylock" is acquired by an
- RCU callback, then a process-context acquisition of this
- lock must use something like spin_lock_irqsave() to
- acquire the lock.
+ Any lock that is acquired within an RCU callback must be acquired
+ elsewhere using an _bh variant of the spinlock primitive.
+ For example, if "mylock" is acquired by an RCU callback, then
+ a process-context acquisition of this lock must use something
+ like spin_lock_bh() to acquire the lock. Please note that
+ it is also OK to use _irq variants of spinlocks, for example,
+ spin_lock_irqsave().
If the process-context code were to simply use spin_lock(),
then, since RCU callbacks can be invoked from softirq context,
