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Date: Mon, 8 Apr 2019 10:52:13 -0700
From: "Paul E. McKenney" <paulmck@...ux.ibm.com>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: linux-kernel@...r.kernel.org, oleg@...hat.com, jannh@...gle.com,
Jonathan Corbet <corbet@....net>,
Josh Triplett <josh@...htriplett.org>,
Lai Jiangshan <jiangshanlai@...il.com>,
linux-doc@...r.kernel.org,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2] doc/rcuref: Document real world examples in kernel
On Sat, Apr 06, 2019 at 02:17:05AM +0000, Joel Fernandes wrote:
> On Thu, Apr 04, 2019 at 01:10:39PM -0700, Paul E. McKenney wrote:
> > 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@...hat.com
> > > Cc: jannh@...gle.com
> > > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.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@...lfernandes.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@...hat.com
> > Cc: jannh@...gle.com
> > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > [ paulmck: Do a bit of wordsmithing. ]
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.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.
>
> This part sounds good to me.
>
> > +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.
> > +
>
> small nit:
> This part is common to both listing B and C right? The delete() is never
> delayed due to the search_and_reference in either case, and the kfree is what
> is delayed. My patch was highlighting the difference between the 2
> listings, but this text says what is common between both listings.
>
> As such I am Ok with the changes you made, and thanks for this document in
> the first place.
Good point! How about the following patch to be merged into the current
patch?
Thanx, Paul
------------------------------------------------------------------------
diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt
index c0bab7fb57e7..5e6429d66c24 100644
--- a/Documentation/RCU/rcuref.txt
+++ b/Documentation/RCU/rcuref.txt
@@ -122,11 +122,12 @@ 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.
+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:
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