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Message-Id: <20180221181410.GL3617@linux.vnet.ibm.com>
Date: Wed, 21 Feb 2018 10:14:10 -0800
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Andrea Parri <parri.andrea@...il.com>
Cc: Alan Stern <stern@...land.harvard.edu>,
Akira Yokosawa <akiyks@...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 Wed, Feb 21, 2018 at 06:58:57PM +0100, Andrea Parri wrote:
> On Wed, Feb 21, 2018 at 12:15:56PM -0500, 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.
> >
> > 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")
>
> Acked-by: Andrea Parri <parri.andrea@...il.com>
Applied, thank you!
Thanx, Paul
> Thanks,
> Andrea
>
>
> >
> > ---
> >
> > 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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