[PATCH tip/core/rcu 1/2] doc/rcuref: Document real world examples in kernel
- Date: Thu, 30 May 2019 07:55:21 -0700
- From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxx>
- Subject: [PATCH tip/core/rcu 1/2] doc/rcuref: Document real world examples in kernel
From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx>
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.
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
[ paulmck: Do a bit of wordsmithing. ]
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
Documentation/RCU/rcuref.txt | 21 ++++++++++++++++++++-
1 file changed, 20 insertions(+), 1 deletion(-)
diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt
index 613033ff2b9b..5e6429d66c24 100644
@@ -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:
@@ -28,7 +29,8 @@ add() search_and_reference()
- atomic_dec(&el->rc, relfunc) ...
+ if(atomic_dec_and_test(&el->rc)) ...
@@ -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:
@@ -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()
+CODE LISTING C:
@@ -114,6 +118,17 @@ 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 clear advantage of both listings B and C over listing A is
+that 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 +145,7 @@ delete()
+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