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Date:   Mon, 21 Oct 2019 19:13:06 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Ingo Molnar <mingo@kernel.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Micah Morton <mortonm@chromium.org>,
        Jann Horn <jannh@google.com>,
        Bart Van Assche <bart.vanassche@wdc.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-security-module <linux-security-module@vger.kernel.org>
Subject: Re: [GIT PULL] SafeSetID LSM changes for 5.4
Message-ID: <20191022021306.GB2479@paulmck-ThinkPad-P72>
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On Mon, Oct 21, 2019 at 08:58:11AM +0200, Ingo Molnar wrote:
> 
> * Paul E. McKenney <paulmck@kernel.org> wrote:
> 
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -383,20 +383,22 @@ do {									      \
> >  } while (0)
> >  
> >  /**
> > - * rcu_swap_protected() - swap an RCU and a regular pointer
> > - * @rcu_ptr: RCU pointer
> > + * rcu_replace() - replace an RCU pointer, returning its old value
> > + * @rcu_ptr: RCU pointer, whose old value is returned
> >   * @ptr: regular pointer
> > - * @c: the conditions under which the dereference will take place
> > + * @c: the lockdep conditions under which the dereference will take place
> >   *
> > - * Perform swap(@rcu_ptr, @ptr) where @rcu_ptr is an RCU-annotated pointer and
> > - * @c is the argument that is passed to the rcu_dereference_protected() call
> > - * used to read that pointer.
> > + * Perform a replacement, where @rcu_ptr is an RCU-annotated
> > + * pointer and @c is the lockdep argument that is passed to the
> > + * rcu_dereference_protected() call used to read that pointer.  The old
> > + * value of @rcu_ptr is returned, and @rcu_ptr is set to @ptr.
> >   */
> > -#define rcu_swap_protected(rcu_ptr, ptr, c) do {			\
> > +#define rcu_replace(rcu_ptr, ptr, c)					\
> > +({									\
> >  	typeof(ptr) __tmp = rcu_dereference_protected((rcu_ptr), (c));	\
> >  	rcu_assign_pointer((rcu_ptr), (ptr));				\
> > -	(ptr) = __tmp;							\
> > -} while (0)
> > +	__tmp;								\
> > +})
> 
> One small suggestion, would it make sense to name it "rcu_replace_pointer()"?
> 
> This would make it fit into the pointer handling family of RCU functions: 
> rcu_assign_pointer(), rcu_access_pointer(), RCU_INIT_POINTER() et al?

Easy enough to make the change.  I will do that tomorrow and test over
the following night.

> rcu_swap() would also look a bit weird if used in MM code. ;-)

How much RCU swap should we configure on this system?  About same amount
as reader-writer swap!  ;-)

							Thanx, Paul