Re: [PATCH v2] doc/rcuref: Document real world examples in kernel

[Paul E. McKenney]

On Fri, Mar 29, 2019 at 10:05:55AM -0400, Joel Fernandes (Google) wrote:
> Document similar real world examples in the kernel corresponding to the
> second and third code snippets. Also correct an issue in
> release_referenced() in the code snippet example.
> 
> Cc: oleg@xxxxxxxxxx
> Cc: jannh@xxxxxxxxxx
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

Good catch, thank you!

As usual, I could not resist doing a bit of wordsmithing.  Please let me
know if I messed anything up in the version shown below.

								Thanx, Paul

------------------------------------------------------------------------

commit adcd92c0ab303b57b28a3cd097bd9ece824c14f6
Author: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Date:   Fri Mar 29 10:05:55 2019 -0400

    doc/rcuref: Document real world examples in kernel
    
    Document similar real world examples in the kernel corresponding to the
    second and third code snippets. Also correct an issue in
    release_referenced() in the code snippet example.
    
    Cc: oleg@xxxxxxxxxx
    Cc: jannh@xxxxxxxxxx
    Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
    [ paulmck: Do a bit of wordsmithing. ]
    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>

diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt
index 613033ff2b9b..c0bab7fb57e7 100644
--- a/Documentation/RCU/rcuref.txt
+++ b/Documentation/RCU/rcuref.txt
@@ -12,6 +12,7 @@ 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()
 {				{
@@ -28,7 +29,8 @@ add()				search_and_reference()
 release_referenced()			delete()
 {					{
     ...					    write_lock(&list_lock);
-    atomic_dec(&el->rc, relfunc)	    ...
+    if(atomic_dec_and_test(&el->rc))	    ...
+	kfree(el);
     ...					    remove_element
 }					    write_unlock(&list_lock);
  					    ...
@@ -44,6 +46,7 @@ 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()
 {					{
@@ -79,6 +82,7 @@ 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()
 {					{
@@ -114,6 +118,16 @@ 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.
 
+A clear advantage of the RCU-based pattern in listing C over the one
+in listing B is that any call to search_and_reference() that locates
+a given object will succeed in obtaining a reference to that object,
+even given a concurrent invocation of delete() for that same object.
+Similarly, a call to delete() is not delayed even if there are an
+arbitrarily large number of calls to search_and_reference() searching
+for the same object that delete() was invoked on.  Instead, all that is
+delayed is 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:
 
@@ -130,3 +144,7 @@ delete()
     	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
+struct posix_acl.