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Date: Mon, 22 Jun 2020 17:09:43 -0700
From: paulmck@...nel.org
To: rcu@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
jiangshanlai@...il.com, dipankar@...ibm.com,
akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
"Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH tip/core/rcu 06/10] docs: RCU: Convert rcuref.txt to ReST
From: Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
- Add a SPDX header;
- Adjust document title;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add it to RCU/index.rst.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
Documentation/RCU/index.rst | 1 +
Documentation/RCU/{rcuref.txt => rcuref.rst} | 199 ++++++++++++++-------------
2 files changed, 104 insertions(+), 96 deletions(-)
rename Documentation/RCU/{rcuref.txt => rcuref.rst} (50%)
diff --git a/Documentation/RCU/index.rst b/Documentation/RCU/index.rst
index 5d5f9a1..9a1d51f 100644
--- a/Documentation/RCU/index.rst
+++ b/Documentation/RCU/index.rst
@@ -18,6 +18,7 @@ RCU concepts
whatisRCU
rcu
rculist_nulls
+ rcuref
torture
listRCU
NMI-RCU
diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.rst
similarity index 50%
rename from Documentation/RCU/rcuref.txt
rename to Documentation/RCU/rcuref.rst
index 5e6429d6..b33aeb1 100644
--- a/Documentation/RCU/rcuref.txt
+++ b/Documentation/RCU/rcuref.rst
@@ -1,4 +1,8 @@
-Reference-count design for elements of lists/arrays protected by RCU.
+.. SPDX-License-Identifier: GPL-2.0
+
+====================================================================
+Reference-count design for elements of lists/arrays protected by RCU
+====================================================================
Please note that the percpu-ref feature is likely your first
@@ -12,32 +16,33 @@ please read on.
Reference counting on elements of lists which are protected by traditional
reader/writer spinlocks or semaphores are straightforward:
-CODE LISTING A:
-1. 2.
-add() search_and_reference()
-{ {
- alloc_object read_lock(&list_lock);
- ... search_for_element
- atomic_set(&el->rc, 1); atomic_inc(&el->rc);
- write_lock(&list_lock); ...
- add_element read_unlock(&list_lock);
- ... ...
- write_unlock(&list_lock); }
-}
-
-3. 4.
-release_referenced() delete()
-{ {
- ... write_lock(&list_lock);
- if(atomic_dec_and_test(&el->rc)) ...
- kfree(el);
- ... remove_element
-} write_unlock(&list_lock);
- ...
- if (atomic_dec_and_test(&el->rc))
- kfree(el);
- ...
- }
+CODE LISTING A::
+
+ 1. 2.
+ add() search_and_reference()
+ { {
+ alloc_object read_lock(&list_lock);
+ ... search_for_element
+ atomic_set(&el->rc, 1); atomic_inc(&el->rc);
+ write_lock(&list_lock); ...
+ add_element read_unlock(&list_lock);
+ ... ...
+ write_unlock(&list_lock); }
+ }
+
+ 3. 4.
+ release_referenced() delete()
+ { {
+ ... write_lock(&list_lock);
+ if(atomic_dec_and_test(&el->rc)) ...
+ kfree(el);
+ ... remove_element
+ } write_unlock(&list_lock);
+ ...
+ if (atomic_dec_and_test(&el->rc))
+ kfree(el);
+ ...
+ }
If this list/array is made lock free using RCU as in changing the
write_lock() in add() and delete() to spin_lock() and changing read_lock()
@@ -46,34 +51,35 @@ search_and_reference() could potentially hold reference to an element which
has already been deleted from the list/array. Use atomic_inc_not_zero()
in this scenario as follows:
-CODE LISTING B:
-1. 2.
-add() search_and_reference()
-{ {
- alloc_object rcu_read_lock();
- ... search_for_element
- atomic_set(&el->rc, 1); if (!atomic_inc_not_zero(&el->rc)) {
- spin_lock(&list_lock); rcu_read_unlock();
- return FAIL;
- add_element }
- ... ...
- spin_unlock(&list_lock); rcu_read_unlock();
-} }
-3. 4.
-release_referenced() delete()
-{ {
- ... spin_lock(&list_lock);
- if (atomic_dec_and_test(&el->rc)) ...
- call_rcu(&el->head, el_free); remove_element
- ... spin_unlock(&list_lock);
-} ...
- if (atomic_dec_and_test(&el->rc))
- call_rcu(&el->head, el_free);
- ...
- }
+CODE LISTING B::
+
+ 1. 2.
+ add() search_and_reference()
+ { {
+ alloc_object rcu_read_lock();
+ ... search_for_element
+ atomic_set(&el->rc, 1); if (!atomic_inc_not_zero(&el->rc)) {
+ spin_lock(&list_lock); rcu_read_unlock();
+ return FAIL;
+ add_element }
+ ... ...
+ spin_unlock(&list_lock); rcu_read_unlock();
+ } }
+ 3. 4.
+ release_referenced() delete()
+ { {
+ ... spin_lock(&list_lock);
+ if (atomic_dec_and_test(&el->rc)) ...
+ call_rcu(&el->head, el_free); remove_element
+ ... spin_unlock(&list_lock);
+ } ...
+ if (atomic_dec_and_test(&el->rc))
+ call_rcu(&el->head, el_free);
+ ...
+ }
Sometimes, a reference to the element needs to be obtained in the
-update (write) stream. In such cases, atomic_inc_not_zero() might be
+update (write) stream. In such cases, atomic_inc_not_zero() might be
overkill, since we hold the update-side spinlock. One might instead
use atomic_inc() in such cases.
@@ -82,39 +88,40 @@ search_and_reference() code path. In such cases, the
atomic_dec_and_test() may be moved from delete() to el_free()
as follows:
-CODE LISTING C:
-1. 2.
-add() search_and_reference()
-{ {
- alloc_object rcu_read_lock();
- ... search_for_element
- atomic_set(&el->rc, 1); atomic_inc(&el->rc);
- spin_lock(&list_lock); ...
-
- add_element rcu_read_unlock();
- ... }
- spin_unlock(&list_lock); 4.
-} delete()
-3. {
-release_referenced() spin_lock(&list_lock);
-{ ...
- ... remove_element
- if (atomic_dec_and_test(&el->rc)) spin_unlock(&list_lock);
- kfree(el); ...
- ... call_rcu(&el->head, el_free);
-} ...
-5. }
-void el_free(struct rcu_head *rhp)
-{
- release_referenced();
-}
+CODE LISTING C::
+
+ 1. 2.
+ add() search_and_reference()
+ { {
+ alloc_object rcu_read_lock();
+ ... search_for_element
+ atomic_set(&el->rc, 1); atomic_inc(&el->rc);
+ spin_lock(&list_lock); ...
+
+ add_element rcu_read_unlock();
+ ... }
+ spin_unlock(&list_lock); 4.
+ } delete()
+ 3. {
+ release_referenced() spin_lock(&list_lock);
+ { ...
+ ... remove_element
+ if (atomic_dec_and_test(&el->rc)) spin_unlock(&list_lock);
+ kfree(el); ...
+ ... call_rcu(&el->head, el_free);
+ } ...
+ 5. }
+ void el_free(struct rcu_head *rhp)
+ {
+ release_referenced();
+ }
The key point is that the initial reference added by add() is not removed
until after a grace period has elapsed following removal. This means that
search_and_reference() cannot find this element, which means that the value
of el->rc cannot increase. Thus, once it reaches zero, there are no
-readers that can or ever will be able to reference the element. The
-element can therefore safely be freed. This in turn guarantees that if
+readers that can or ever will be able to reference the element. The
+element can therefore safely be freed. This in turn guarantees that if
any reader finds the element, that reader may safely acquire a reference
without checking the value of the reference counter.
@@ -130,21 +137,21 @@ the eventual invocation of kfree(), which is usually not a problem on
modern computer systems, even the small ones.
In cases where delete() can sleep, synchronize_rcu() can be called from
-delete(), so that el_free() can be subsumed into delete as follows:
-
-4.
-delete()
-{
- spin_lock(&list_lock);
- ...
- remove_element
- spin_unlock(&list_lock);
- ...
- synchronize_rcu();
- if (atomic_dec_and_test(&el->rc))
- kfree(el);
- ...
-}
+delete(), so that el_free() can be subsumed into delete as follows::
+
+ 4.
+ delete()
+ {
+ spin_lock(&list_lock);
+ ...
+ remove_element
+ spin_unlock(&list_lock);
+ ...
+ synchronize_rcu();
+ if (atomic_dec_and_test(&el->rc))
+ kfree(el);
+ ...
+ }
As additional examples in the kernel, the pattern in listing C is used by
reference counting of struct pid, while the pattern in listing B is used by
--
2.9.5
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