linux-kernel - Re: [PATCH] files: rcu free files

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Message-ID: <20201209232618.GK2657@paulmck-ThinkPad-P72>
Date:   Wed, 9 Dec 2020 15:26:18 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jann@...jh.net>
Subject: Re: [PATCH] files: rcu free files_struct

On Wed, Dec 09, 2020 at 11:07:55PM +0000, Matthew Wilcox wrote:
> On Wed, Dec 09, 2020 at 03:01:36PM -0800, Linus Torvalds wrote:
> > On Wed, Dec 9, 2020 at 2:58 PM Al Viro <viro@...iv.linux.org.uk> wrote:
> > >
> > > On Wed, Dec 09, 2020 at 07:49:38PM +0000, Matthew Wilcox wrote:
> > > >
> > > > Assuming this is safe, you can use RCU_INIT_POINTER() here because you're
> > > > storing NULL, so you don't need the wmb() before storing the pointer.
> > >
> > > fs/file.c:pick_file() would make more interesting target for the same treatment...
> > 
> > Actually, don't.
> > 
> > rcu_assign_pointer() itself already does the optimization for the case
> > of a constant NULL pointer assignment.
> > 
> > So there's no need to manually change things to RCU_INIT_POINTER().
> 
> I missed that, and the documentation wasn't updated by
> 3a37f7275cda5ad25c1fe9be8f20c76c60d175fa.

Can't trust the author of that patch!  ;-)

> Paul, how about this?
> 
> +++ b/Documentation/RCU/Design/Requirements/Requirements.rst
> @@ -1668,8 +1668,10 @@ against mishaps and misuse:
>     this purpose.
>  #. It is not necessary to use rcu_assign_pointer() when creating
>     linked structures that are to be published via a single external
> -   pointer. The RCU_INIT_POINTER() macro is provided for this task
> -   and also for assigning ``NULL`` pointers at runtime.
> +   pointer. The RCU_INIT_POINTER() macro is provided for this task.
> +   It used to be more efficient to use RCU_INIT_POINTER() to store a
> +   ``NULL`` pointer, but rcu_assign_pointer() now optimises for a constant
> +   ``NULL`` pointer itself.
>  
>  This not a hard-and-fast list: RCU's diagnostic capabilities will
>  continue to be guided by the number and type of usage bugs found in

Looks good to me!  If you send a complete patch, I will be happy to pull
it in.

							Thanx, Paul