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Date:   Sun, 25 Feb 2018 14:33:14 -0800
From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     Akira Yokosawa <akiyks@gmail.com>
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        Andrea Parri <parri.andrea@gmail.com>,
        Kernel development list <linux-kernel@vger.kernel.org>,
        mingo@kernel.org, Will Deacon <will.deacon@arm.com>,
        peterz@infradead.org, boqun.feng@gmail.com, npiggin@gmail.com,
        dhowells@redhat.com, Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Patrick Bellasi <patrick.bellasi@arm.com>
Subject: Re: [PATCH] tools/memory-model: update: remove rb-dep,
 smp_read_barrier_depends, and lockless_dereference
Reply-To: paulmck@linux.vnet.ibm.com
References: <20180224143658.GA3556@andrea>
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 <b01f23b8-058c-a968-2b09-37f315e76681@gmail.com>
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On Sun, Feb 25, 2018 at 07:47:23AM +0900, Akira Yokosawa wrote:
> On 2018/02/24 10:08:14 -0800, Paul E. McKenney wrote:
> > On Sat, Feb 24, 2018 at 11:49:20AM -0500, Alan Stern wrote:
> >> On Sat, 24 Feb 2018, Andrea Parri wrote:
> >>
> >>> On Fri, Feb 23, 2018 at 07:30:13PM -0800, Paul E. McKenney wrote:
> >>>> On Sat, Feb 24, 2018 at 12:22:24PM +0900, Akira Yokosawa wrote:
> >>>>> On 2018/02/22 07:29:02 +0900, Akira Yokosawa wrote:
> >>>>>> On 2018/02/22 2:15, Alan Stern wrote:
> >>>
> >>> [...]
> >>>
> >>>>>>>
> >>>>>>> Akira pointed out some typos in the original patch, and he noted that
> >>>>>>> cheatsheet.txt should be updated to indicate how unsuccessful RMW
> >>>>>>> operations relate to address dependencies.
> >>>>>>
> >>>>>> My point was to separate unannotated loads from READ_ONCE(), if the
> >>>>>> cheatsheet should concern such accesses as well.
> >>>>>> Unsuccessful RMW operations were brought up by Andrea.
> >>>>>>
> >>>>>
> >>>>> Paul, can you amend above paragraph in the change log to something like:
> >>>>>
> >>>>>     Akira pointed out some typos in the original patch, and he noted that
> >>>>>     cheatsheet.txt should be updated to indicate READ_ONCE() implies
> >>>>>     address dependency, which invited Andrea's observation that it should
> >>>>>     also be updated to indicate how unsuccessful RMW operations relate to
> >>>>>     address dependencies.
> >>>>>
> >>>>> , if Alan and Andrea are OK with the amendment.
> >>>>>
> >>>>> Also, please append my Acked-by.
> >>>>>
> >>>>> Acked-by: Akira Yokosawa <akiyks@gmail.com>
> >>>>
> >>>> I can still amend this, and have added your Acked-by.  If Alan and Andrea
> >>>> OK with your change, I will apply that also.
> >>>
> >>> LGTM. Thanks,
> >>
> >> Me too.
> > 
> > Very good, how about this for the new version?
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > commit 21ede43970e50b7397420f17ed08bb02c187e2eb
> > Author: Alan Stern <stern@rowland.harvard.edu>
> > Date:   Wed Feb 21 12:15:56 2018 -0500
> > 
> >     tools/memory-model: Update: Remove rb-dep, smp_read_barrier_depends, and lockless_dereference
> >     
> >     Commit bf28ae562744 ("tools/memory-model: Remove rb-dep,
> >     smp_read_barrier_depends, and lockless_dereference") was accidentally
> >     merged too early, while it was still in RFC form.  This patch adds in
> >     the missing pieces.
> >     
> >     Akira pointed out some typos in the original patch, and he noted that
> >     cheatsheet.txt should indicate that READ_ONCE() now implies an address
> >     dependency.  Andrea suggested documenting the relationship betwwen
> >     unsuccessful RMW operations and address dependencies.
> 
> Looks good. But I've found a remaining typo in the patch. See below.
> 
> >     >     Andrea pointed out that the macro for rcu_dereference() in linux.def
> >     should now use the "once" annotation instead of "deref".  He also
> >     suggested that the comments should mention commit 5a8897cc7631
> >     ("locking/atomics/alpha: Add smp_read_barrier_depends() to
> >     _release()/_relaxed() atomics") as an important precursor, and he
> >     contributed commit cb13b424e986 ("locking/xchg/alpha: Add
> >     unconditional memory barrier to cmpxchg()"), another prerequisite.
> >     
> >     Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
> >     Suggested-by: Akira Yokosawa <akiyks@gmail.com>
> >     Suggested-by: Andrea Parri <parri.andrea@gmail.com>
> >     Fixes: bf28ae562744 ("tools/memory-model: Remove rb-dep, smp_read_barrier_depends, and lockless_dereference")
> >     Acked-by: Andrea Parri <parri.andrea@gmail.com>
> >     Acked-by: Akira Yokosawa <akiyks@gmail.com>
> >     Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > 
> > diff --git a/tools/memory-model/Documentation/cheatsheet.txt b/tools/memory-model/Documentation/cheatsheet.txt
> > index 04e458acd6d4..956b1ae4aafb 100644
> > --- a/tools/memory-model/Documentation/cheatsheet.txt
> > +++ b/tools/memory-model/Documentation/cheatsheet.txt
> > @@ -1,11 +1,11 @@
> >                                    Prior Operation     Subsequent Operation
> >                                    ---------------  ---------------------------
> >                                 C  Self  R  W  RWM  Self  R  W  DR  DW  RMW  SV
> > -                              __  ----  -  -  ---  ----  -  -  --  --  ---  --
> > +                              --  ----  -  -  ---  ----  -  -  --  --  ---  --
> >  
> >  Store, e.g., WRITE_ONCE()            Y                                       Y
> > -Load, e.g., READ_ONCE()              Y                              Y        Y
> > -Unsuccessful RMW operation           Y                              Y        Y
> > +Load, e.g., READ_ONCE()              Y                          Y   Y        Y
> > +Unsuccessful RMW operation           Y                          Y   Y        Y
> >  rcu_dereference()                    Y                          Y   Y        Y
> >  Successful *_acquire()               R                   Y  Y   Y   Y    Y   Y
> >  Successful *_release()         C        Y  Y    Y     W                      Y
> > diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt
> > index dae8b8cb2ad3..889fabef7d83 100644
> > --- a/tools/memory-model/Documentation/explanation.txt
> > +++ b/tools/memory-model/Documentation/explanation.txt
> > @@ -826,7 +826,7 @@ A-cumulative; they only affect the propagation of stores that are
> >  executed on C before the fence (i.e., those which precede the fence in
> >  program order).
> >  
> > -read_lock(), rcu_read_unlock(), and synchronize_rcu() fences have
> > +read_read_lock(), rcu_read_unlock(), and synchronize_rcu() fences have
> 
>    rcu_read_lock()
> 
> Don't ask why I missed this in the first place...
> 
> Paul, can you fix this directly?

Done!

								Thanx, Paul

>         Thanks, Akira
>         
> >  other properties which we discuss later.
> >  
> >  
> > @@ -1138,7 +1138,7 @@ final effect is that even though the two loads really are executed in
> >  program order, it appears that they aren't.
> >  
> >  This could not have happened if the local cache had processed the
> > -incoming stores in FIFO order.  In constrast, other architectures
> > +incoming stores in FIFO order.  By contrast, other architectures
> >  maintain at least the appearance of FIFO order.
> >  
> >  In practice, this difficulty is solved by inserting a special fence
> > diff --git a/tools/memory-model/linux-kernel.def b/tools/memory-model/linux-kernel.def
> > index 5dfb9c7f3462..397e4e67e8c8 100644
> > --- a/tools/memory-model/linux-kernel.def
> > +++ b/tools/memory-model/linux-kernel.def
> > @@ -13,7 +13,7 @@ WRITE_ONCE(X,V) { __store{once}(X,V); }
> >  smp_store_release(X,V) { __store{release}(*X,V); }
> >  smp_load_acquire(X) __load{acquire}(*X)
> >  rcu_assign_pointer(X,V) { __store{release}(X,V); }
> > -rcu_dereference(X) __load{deref}(X)
> > +rcu_dereference(X) __load{once}(X)
> >  
> >  // Fences
> >  smp_mb() { __fence{mb} ; }
> > 
>