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doc: Make whatisRCU.rst note that spinlocks are RCU readers

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In kernels built with CONFIG_PREEMPT_RT=n, spinlock critical sections
are RCU readers because they disable preemption.  However, they are also
RCU readers in CONFIG_PREEMPT_RT=y because in that case the locking
primitives contain rcu_read_lock() and rcu_read_unlock().  Therefore,
upgrade whatisRCU.rst to document this non-obvious case.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
This commit is contained in:
Paul E. McKenney 2023-11-24 15:06:46 -08:00 committed by Boqun Feng
parent 120311acb0
commit 8dbc33b4d1
1 changed files with 13 additions and 2 deletions
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15
Documentation/RCU/whatisRCU.rst
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@ -172,14 +172,25 @@ rcu_read_lock()
critical section. Reference counts may be used in conjunction
with RCU to maintain longer-term references to data structures.
Note that anything that disables bottom halves, preemption,
or interrupts also enters an RCU read-side critical section.
Acquiring a spinlock also enters an RCU read-side critical
sections, even for spinlocks that do not disable preemption,
as is the case in kernels built with CONFIG_PREEMPT_RT=y.
Sleeplocks do *not* enter RCU read-side critical sections.
rcu_read_unlock()
^^^^^^^^^^^^^^^^^
void rcu_read_unlock(void);
This temporal primitives is used by a reader to inform the
reclaimer that the reader is exiting an RCU read-side critical
section. Note that RCU read-side critical sections may be nested
and/or overlapping.
section. Anything that enables bottom halves, preemption,
or interrupts also exits an RCU read-side critical section.
Releasing a spinlock also exits an RCU read-side critical section.
Note that RCU read-side critical sections may be nested and/or
overlapping.
synchronize_rcu()
^^^^^^^^^^^^^^^^^