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

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