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@xxxxxxxxxx
> > Cc: jannh@xxxxxxxxxx
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> 
> 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@xxxxxxxxxxxxxxxxx>
> 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@xxxxxxxxxx
>     Cc: jannh@xxxxxxxxxx
>     Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
>     [ paulmck: Do a bit of wordsmithing. ]
>     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
> 
> 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.
>