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-rw-r--r--Documentation/RCU/whatisRCU.rst19
1 files changed, 15 insertions, 4 deletions
diff --git a/Documentation/RCU/whatisRCU.rst b/Documentation/RCU/whatisRCU.rst
index 60ce02475142..872ac665223f 100644
--- a/Documentation/RCU/whatisRCU.rst
+++ b/Documentation/RCU/whatisRCU.rst
@@ -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()
^^^^^^^^^^^^^^^^^
@@ -952,8 +963,8 @@ unfortunately any spinlock in a ``SLAB_TYPESAFE_BY_RCU`` object must be
initialized after each and every call to kmem_cache_alloc(), which renders
reference-free spinlock acquisition completely unsafe. Therefore, when
using ``SLAB_TYPESAFE_BY_RCU``, make proper use of a reference counter.
-(Those willing to use a kmem_cache constructor may also use locking,
-including cache-friendly sequence locking.)
+(Those willing to initialize their locks in a kmem_cache constructor
+may also use locking, including cache-friendly sequence locking.)
With traditional reference counting -- such as that implemented by the
kref library in Linux -- there is typically code that runs when the last