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

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

[Joel Fernandes (Google)]

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>

v1->v2:
 - minor fixups, label code listings.

 Documentation/RCU/rcuref.txt | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt
index 613033ff2b9b..a49d525ce975 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 +118,13 @@ 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.
 
+As can be seen, a clear advantage of the pattern in listing C is, if there are
+several calls to search_and_reference() in parallel to the delete(), then all
+of those will succeed in obtaining a reference to the object if the object
+could be found in the list before it was deleted in delete(), unlike the
+pattern in listing B which would fail to acquire references in such a situation
+even though the object is still in memory.
+
 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 +141,7 @@ 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.
-- 
2.21.0.392.gf8f6787159e-goog

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@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.

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

[Joel Fernandes]

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.

thanks,

- Joel



>  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.
> 

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

[Paul E. McKenney]

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:

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

[Joel Fernandes]

On Mon, Apr 08, 2019 at 10:52:13AM -0700, Paul E. McKenney wrote:
> 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:
> 

This one looks better to me, thanks a lot!

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

thanks,

- Joel

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

[Paul E. McKenney]

On Mon, Apr 08, 2019 at 02:08:59PM -0400, Joel Fernandes wrote:
> On Mon, Apr 08, 2019 at 10:52:13AM -0700, Paul E. McKenney wrote:
> > 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:
> 
> This one looks better to me, thanks a lot!
> 
> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

Thank you!  (Though this one gets merged into your original patch.)

							Thanx, Paul