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: [email protected]
Cc: [email protected]
Signed-off-by: Joel Fernandes (Google) <[email protected]>
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
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: [email protected]
> Cc: [email protected]
> Signed-off-by: Joel Fernandes (Google) <[email protected]>
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) <[email protected]>
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: [email protected]
Cc: [email protected]
Signed-off-by: Joel Fernandes (Google) <[email protected]>
[ paulmck: Do a bit of wordsmithing. ]
Signed-off-by: Paul E. McKenney <[email protected]>
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.
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: [email protected]
> > Cc: [email protected]
> > Signed-off-by: Joel Fernandes (Google) <[email protected]>
>
> 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) <[email protected]>
> 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: [email protected]
> Cc: [email protected]
> Signed-off-by: Joel Fernandes (Google) <[email protected]>
> [ paulmck: Do a bit of wordsmithing. ]
> Signed-off-by: Paul E. McKenney <[email protected]>
>
> 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.
>
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: [email protected]
> > > > > Cc: [email protected]
> > > > > Signed-off-by: Joel Fernandes (Google) <[email protected]>
> > > >
> > > > 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) <[email protected]>
> > > > 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: [email protected]
> > > > Cc: [email protected]
> > > > Signed-off-by: Joel Fernandes (Google) <[email protected]>
> > > > [ paulmck: Do a bit of wordsmithing. ]
> > > > Signed-off-by: Paul E. McKenney <[email protected]>
> > > >
> > > > 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) <[email protected]>
Thank you! (Though this one gets merged into your original patch.)
Thanx, Paul
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: [email protected]
> > > Cc: [email protected]
> > > Signed-off-by: Joel Fernandes (Google) <[email protected]>
> >
> > 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) <[email protected]>
> > 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: [email protected]
> > Cc: [email protected]
> > Signed-off-by: Joel Fernandes (Google) <[email protected]>
> > [ paulmck: Do a bit of wordsmithing. ]
> > Signed-off-by: Paul E. McKenney <[email protected]>
> >
> > 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:
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: [email protected]
> > > > Cc: [email protected]
> > > > Signed-off-by: Joel Fernandes (Google) <[email protected]>
> > >
> > > 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) <[email protected]>
> > > 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: [email protected]
> > > Cc: [email protected]
> > > Signed-off-by: Joel Fernandes (Google) <[email protected]>
> > > [ paulmck: Do a bit of wordsmithing. ]
> > > Signed-off-by: Paul E. McKenney <[email protected]>
> > >
> > > 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) <[email protected]>
thanks,
- Joel