by Andrea Parri

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2018-02-25 22:33:45

by Paul E. McKenney

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Subject: Re: [PATCH] tools/memory-model: update: remove rb-dep, smp_read_barrier_depends, and lockless_dereference

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 <[email protected]>
> >>>>
> >>>> 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 <[email protected]>
> > 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 <[email protected]>
> > Suggested-by: Akira Yokosawa <[email protected]>
> > Suggested-by: Andrea Parri <[email protected]>
> > Fixes: bf28ae562744 ("tools/memory-model: Remove rb-dep, smp_read_barrier_depends, and lockless_dereference")
> > Acked-by: Andrea Parri <[email protected]>
> > Acked-by: Akira Yokosawa <[email protected]>
> > Signed-off-by: Paul E. McKenney <[email protected]>
> >
> > 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} ; }
> >
>