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doc: RCU callback locks need only _bh, not necessarily _irq

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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>
This commit is contained in:
Jiunn Chang 2019-06-27 16:01:47 -05:00 committed by Jonathan Corbet
parent 62ee81b568
commit acb6258acc
1 changed files with 7 additions and 6 deletions
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13
Documentation/RCU/UP.rst
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@ -113,12 +113,13 @@ Answer to Quick Quiz #1:
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,