docs: RCU: Convert rcuref.txt to ReST

- 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@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

2 files changed, 104 insertions, 96 deletions

diff --git a/Documentation/RCU/index.rst b/Documentation/RCU/index.rst
index 5d5f9a1ab8f9..9a1d51f394dc 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
index 5e6429d66c24..b33aeb14fde3 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