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SubjectRe: [PATCH v2] doc/rcuref: Document real world examples in kernel
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@redhat.com
> Cc: jannh@google.com
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

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@joelfernandes.org>
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@redhat.com
Cc: jannh@google.com
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
[ paulmck: Do a bit of wordsmithing. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

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.
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