doc: Add refcount analogy to What is RCU

The reader-writer-lock analogy is a useful way to think about RCU, but
it is not always applicable.  It is useful to have other analogies to
work with, and particularly to emphasise that no single analogy is
perfect.

This patch add a "RCU as reference count" to the "what is RCU" document.

See https://lwn.net/Articles/872559/

[ paulmck: Apply Akira Yokosawa feedback. ]
Reviewed-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
NeilBrown 2021-10-26 15:53:38 +11:00 committed by Paul E. McKenney
parent db4cb76861
commit 7c0be9f890

View File

@ -39,9 +39,11 @@ different paths, as follows:
:ref:`6. ANALOGY WITH READER-WRITER LOCKING <6_whatisRCU>`
:ref:`7. FULL LIST OF RCU APIs <7_whatisRCU>`
:ref:`7. ANALOGY WITH REFERENCE COUNTING <7_whatisRCU>`
:ref:`8. ANSWERS TO QUICK QUIZZES <8_whatisRCU>`
:ref:`8. FULL LIST OF RCU APIs <8_whatisRCU>`
:ref:`9. ANSWERS TO QUICK QUIZZES <9_whatisRCU>`
People who prefer starting with a conceptual overview should focus on
Section 1, though most readers will profit by reading this section at
@ -677,7 +679,7 @@ Quick Quiz #1:
occur when using this algorithm in a real-world Linux
kernel? How could this deadlock be avoided?
:ref:`Answers to Quick Quiz <8_whatisRCU>`
:ref:`Answers to Quick Quiz <9_whatisRCU>`
5B. "TOY" EXAMPLE #2: CLASSIC RCU
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@ -732,7 +734,7 @@ Quick Quiz #2:
Give an example where Classic RCU's read-side
overhead is **negative**.
:ref:`Answers to Quick Quiz <8_whatisRCU>`
:ref:`Answers to Quick Quiz <9_whatisRCU>`
.. _quiz_3:
@ -741,7 +743,7 @@ Quick Quiz #3:
critical section, what the heck do you do in
CONFIG_PREEMPT_RT, where normal spinlocks can block???
:ref:`Answers to Quick Quiz <8_whatisRCU>`
:ref:`Answers to Quick Quiz <9_whatisRCU>`
.. _6_whatisRCU:
@ -872,7 +874,79 @@ be used in place of synchronize_rcu().
.. _7_whatisRCU:
7. FULL LIST OF RCU APIs
7. ANALOGY WITH REFERENCE COUNTING
-----------------------------------
The reader-writer analogy (illustrated by the previous section) is not
always the best way to think about using RCU. Another helpful analogy
considers RCU an effective reference count on everything which is
protected by RCU.
A reference count typically does not prevent the referenced object's
values from changing, but does prevent changes to type -- particularly the
gross change of type that happens when that object's memory is freed and
re-allocated for some other purpose. Once a type-safe reference to the
object is obtained, some other mechanism is needed to ensure consistent
access to the data in the object. This could involve taking a spinlock,
but with RCU the typical approach is to perform reads with SMP-aware
operations such as smp_load_acquire(), to perform updates with atomic
read-modify-write operations, and to provide the necessary ordering.
RCU provides a number of support functions that embed the required
operations and ordering, such as the list_for_each_entry_rcu() macro
used in the previous section.
A more focused view of the reference counting behavior is that,
between rcu_read_lock() and rcu_read_unlock(), any reference taken with
rcu_dereference() on a pointer marked as ``__rcu`` can be treated as
though a reference-count on that object has been temporarily increased.
This prevents the object from changing type. Exactly what this means
will depend on normal expectations of objects of that type, but it
typically includes that spinlocks can still be safely locked, normal
reference counters can be safely manipulated, and ``__rcu`` pointers
can be safely dereferenced.
Some operations that one might expect to see on an object for
which an RCU reference is held include:
- Copying out data that is guaranteed to be stable by the object's type.
- Using kref_get_unless_zero() or similar to get a longer-term
reference. This may fail of course.
- Acquiring a spinlock in the object, and checking if the object still
is the expected object and if so, manipulating it freely.
The understanding that RCU provides a reference that only prevents a
change of type is particularly visible with objects allocated from a
slab cache marked ``SLAB_TYPESAFE_BY_RCU``. RCU operations may yield a
reference to an object from such a cache that has been concurrently
freed and the memory reallocated to a completely different object,
though of the same type. In this case RCU doesn't even protect the
identity of the object from changing, only its type. So the object
found may not be the one expected, but it will be one where it is safe
to take a reference or spinlock and then confirm that the identity
matches the expectations.
With traditional reference counting -- such as that implemented by the
kref library in Linux -- there is typically code that runs when the last
reference to an object is dropped. With kref, this is the function
passed to kref_put(). When RCU is being used, such finalization code
must not be run until all ``__rcu`` pointers referencing the object have
been updated, and then a grace period has passed. Every remaining
globally visible pointer to the object must be considered to be a
potential counted reference, and the finalization code is typically run
using call_rcu() only after all those pointers have been changed.
To see how to choose between these two analogies -- of RCU as a
reader-writer lock and RCU as a reference counting system -- it is useful
to reflect on the scale of the thing being protected. The reader-writer
lock analogy looks at larger multi-part objects such as a linked list
and shows how RCU can facilitate concurrency while elements are added
to, and removed from, the list. The reference-count analogy looks at
the individual objects and looks at how they can be accessed safely
within whatever whole they are a part of.
.. _8_whatisRCU:
8. FULL LIST OF RCU APIs
-------------------------
The RCU APIs are documented in docbook-format header comments in the
@ -1035,9 +1109,9 @@ g. Otherwise, use RCU.
Of course, this all assumes that you have determined that RCU is in fact
the right tool for your job.
.. _8_whatisRCU:
.. _9_whatisRCU:
8. ANSWERS TO QUICK QUIZZES
9. ANSWERS TO QUICK QUIZZES
----------------------------
Quick Quiz #1: