linux-kernel - Re: [PATCH v4 1/3] locking: Document the semantics of spin_is

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Date:   Sun, 8 Apr 2018 14:32:53 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     paulmck@...ux.vnet.ibm.com,
        Andrea Parri <andrea.parri@...rulasolutions.com>
Cc:     linux-kernel@...r.kernel.org,
        Alan Stern <stern@...land.harvard.edu>,
        David Howells <dhowells@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v4 1/3] locking: Document the semantics of
 spin_is_locked()

On 04/08/2018 02:14 PM, Paul E. McKenney wrote:
> On Fri, Apr 06, 2018 at 11:58:25PM +0200, Andrea Parri wrote:
>> On Fri, Apr 06, 2018 at 02:08:16PM -0700, Randy Dunlap wrote:
>>> On 04/06/2018 02:07 PM, Paul E. McKenney wrote:
>>>> On Fri, Apr 06, 2018 at 02:01:41PM -0700, Randy Dunlap wrote:
>>>>> On 04/06/2018 12:47 PM, Andrea Parri wrote:
>>>>>> There appeared to be a certain, recurrent uncertainty concerning the
>>>>>> semantics of spin_is_locked(), likely a consequence of the fact that
>>>>>> this semantics remains undocumented or that it has been historically
>>>>>> linked to the (likewise unclear) semantics of spin_unlock_wait().
>>>>>>
>>>>>> A recent auditing [1] of the callers of the primitive confirmed that
>>>>>> none of them are relying on particular ordering guarantees; document
>>>>>> this semantics by adding a docbook header to spin_is_locked(). Also,
>>>>>> describe behaviors specific to certain CONFIG_SMP=n builds.
>>>>>>
>>>>>> [1] https://marc.info/?l=linux-kernel&m=151981440005264&w=2
>>>>>>     https://marc.info/?l=linux-kernel&m=152042843808540&w=2
>>>>>>     https://marc.info/?l=linux-kernel&m=152043346110262&w=2
>>>>>>
>>>>>> Co-Developed-by: Andrea Parri <andrea.parri@...rulasolutions.com>
>>>>>> Co-Developed-by: Alan Stern <stern@...land.harvard.edu>
>>>>>> Co-Developed-by: David Howells <dhowells@...hat.com>
>>>>>> Signed-off-by: Andrea Parri <andrea.parri@...rulasolutions.com>
>>>>>> Signed-off-by: Alan Stern <stern@...land.harvard.edu>
>>>>>> Signed-off-by: David Howells <dhowells@...hat.com>
>>>>>> Cc: Will Deacon <will.deacon@....com>
>>>>>> Cc: Peter Zijlstra <peterz@...radead.org>
>>>>>> Cc: Boqun Feng <boqun.feng@...il.com>
>>>>>> Cc: Nicholas Piggin <npiggin@...il.com>
>>>>>> Cc: Jade Alglave <j.alglave@....ac.uk>
>>>>>> Cc: Luc Maranget <luc.maranget@...ia.fr>
>>>>>> Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
>>>>>> Cc: Akira Yokosawa <akiyks@...il.com>
>>>>>> Cc: Ingo Molnar <mingo@...nel.org>
>>>>>> ---
>>>>>>  include/linux/spinlock.h | 18 ++++++++++++++++++
>>>>>>  1 file changed, 18 insertions(+)
>>>>>>
>>>>>> diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
>>>>>> index 4894d322d2584..1e8a464358384 100644
>>>>>> --- a/include/linux/spinlock.h
>>>>>> +++ b/include/linux/spinlock.h
>>>>>> @@ -380,6 +380,24 @@ static __always_inline int spin_trylock_irq(spinlock_t *lock)
>>>>>>  	raw_spin_trylock_irqsave(spinlock_check(lock), flags); \
>>>>>>  })
>>>>>>  
>>>>>> +/**
>>>>>> + * spin_is_locked() - Check whether a spinlock is locked.
>>>>>> + * @lock: Pointer to the spinlock.
>>>>>> + *
>>>>>> + * This function is NOT required to provide any memory ordering
>>>>>> + * guarantees; it could be used for debugging purposes or, when
>>>>>> + * additional synchronization is needed, accompanied with other
>>>>>> + * constructs (memory barriers) enforcing the synchronization.
>>>>>> + *
>>>>>> + * Returns: 1 if @lock is locked, 0 otherwise.
>>>>>
>>>>> Sorry, minor nit:
>>>>> s/Returns:/Return:/
>>>>> (according to kernel-doc.rst)
>>>>>
>>>>> although I agree that "Returns:" is better.
>>>>> [I should have changed that years ago.]
>>>>
>>>> Agreed, English grammar and templates often seem to conflict.
>>>>
>>>> So should we change this comment, or are you instead proposing to add
>>>> "Returns:" as valid kernel-doc?
>>>
>>> Please change this patch to current doc syntax.
>>> Any changes to kernel-doc syntax would come later.
> 
> Are you sure?
> 
> $ git grep "\* Returns:" | wc -l
> 2470
> $ git grep "\* Return:" | wc -l
> 4144
> 
> Looks like more than a third of them are already "Returns:".  ;-)
> 
>> Paul: I understand that you're going to do this change "in place"; please
>> let me know if I'm wrong/if you need a new submission.
> 
> If Randy is certain that he would like to continue propagating
> this grammatical infelicity, sure.  ;-)

eh?
Well, Documentation/doc-guide/kernel-doc.rst says "Return:", but it appears
that it does not matter to scripts/kernel-doc -- it's just the name of a
"section" of the documentation and can be spelled any way!  oh well.  :)

Acked-by: Randy Dunlap <rdunlap@...radead.org>

Thanks.

-- 
~Randy