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Date:   Fri, 23 Feb 2018 19:30:13 -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
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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:
> >> 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 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.

							Thanx, Paul

> >> 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")
> >>
> > 
> > The change itself looks good to me.
> > 
> > Acked-by: Akira Yokosawa <akiyks@gmail.com>
> > 
> >        Thanks, Akira
> > 
> >> ---
> >>
> >>  tools/memory-model/Documentation/cheatsheet.txt  |    6 +++---
> >>  tools/memory-model/Documentation/explanation.txt |    4 ++--
> >>  tools/memory-model/linux-kernel.def              |    2 +-
> >>  3 files changed, 6 insertions(+), 6 deletions(-)
> >>
> >> Index: usb-4.x/tools/memory-model/Documentation/cheatsheet.txt
> >> ===================================================================
> >> --- usb-4.x.orig/tools/memory-model/Documentation/cheatsheet.txt
> >> +++ usb-4.x/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
> >> Index: usb-4.x/tools/memory-model/Documentation/explanation.txt
> >> ===================================================================
> >> --- usb-4.x.orig/tools/memory-model/Documentation/explanation.txt
> >> +++ usb-4.x/tools/memory-model/Documentation/explanation.txt
> >> @@ -826,7 +826,7 @@ A-cumulative; they only affect the propa
> >>  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
> >>  other properties which we discuss later.
> >>  
> >>  
> >> @@ -1138,7 +1138,7 @@ final effect is that even though the two
> >>  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
> >> Index: usb-4.x/tools/memory-model/linux-kernel.def
> >> ===================================================================
> >> --- usb-4.x.orig/tools/memory-model/linux-kernel.def
> >> +++ usb-4.x/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} ; }
> >>
> > 
>