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Date: Fri, 23 Feb 2018 19:30:13 -0800
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Akira Yokosawa <akiyks@...il.com>
Cc: Alan Stern <stern@...land.harvard.edu>,
Andrea Parri <parri.andrea@...il.com>,
Kernel development list <linux-kernel@...r.kernel.org>,
mingo@...nel.org, Will Deacon <will.deacon@....com>,
peterz@...radead.org, boqun.feng@...il.com, npiggin@...il.com,
dhowells@...hat.com, Jade Alglave <j.alglave@....ac.uk>,
Luc Maranget <luc.maranget@...ia.fr>,
Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: [PATCH] tools/memory-model: update: remove rb-dep,
smp_read_barrier_depends, and lockless_dereference
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@...il.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@...land.harvard.edu>
> >> Suggested-by: Akira Yokosawa <akiyks@...il.com>
> >> Suggested-by: Andrea Parri <parri.andrea@...il.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@...il.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} ; }
> >>
> >
>
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