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

[Akira Yokosawa]

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@xxxxxxxxx>

        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@xxxxxxxxxxxxxxxxxxx>
>> Suggested-by: Akira Yokosawa <akiyks@xxxxxxxxx>
>> Suggested-by: Andrea Parri <parri.andrea@xxxxxxxxx>
>> 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@xxxxxxxxx>
> 
>        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} ; }
>>
>