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Date: Sat, 24 Feb 2018 12:22:24 +0900
From: Akira Yokosawa <akiyks@...il.com>
To: Alan Stern <stern@...land.harvard.edu>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: 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>,
Akira Yokosawa <akiyks@...il.com>
Subject: Re: [PATCH] tools/memory-model: update: remove rb-dep,
smp_read_barrier_depends, and lockless_dereference
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>
Thanks, Akira
>>
>> 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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