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Date:   Mon, 8 Apr 2019 10:52:13 -0700
From:   "Paul E. McKenney" <paulmck@linux.ibm.com>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     linux-kernel@vger.kernel.org, oleg@redhat.com, jannh@google.com,
        Jonathan Corbet <corbet@lwn.net>,
        Josh Triplett <josh@joshtriplett.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        linux-doc@vger.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH v2] doc/rcuref: Document real world examples in kernel
Reply-To: paulmck@linux.ibm.com
References: <20190329140555.118463-1-joel@joelfernandes.org>
 <20190404201039.GL14111@linux.ibm.com>
 <20190406021705.GA6615@localhost>
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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@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.
> 
> 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: