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Subject: Re: [PATCH v4 1/3] locking: Document the semantics of
 spin_is_locked()
To:     paulmck@linux.vnet.ibm.com
Cc:     Andrea Parri <andrea.parri@amarulasolutions.com>,
        linux-kernel@vger.kernel.org,
        Alan Stern <stern@rowland.harvard.edu>,
        David Howells <dhowells@redhat.com>,
        Will Deacon <will.deacon@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Akira Yokosawa <akiyks@gmail.com>,
        Ingo Molnar <mingo@kernel.org>
References: <1522600912-6208-2-git-send-email-andrea.parri@amarulasolutions.com>
 <1523044060-5568-1-git-send-email-andrea.parri@amarulasolutions.com>
 <b0dbf383-6403-a408-047d-0dd0d2135a03@infradead.org>
 <20180406210741.GF3948@linux.vnet.ibm.com>
From:   Randy Dunlap <rdunlap@infradead.org>
Message-ID: <30d15756-1cb4-7123-b4d9-ce2e66861df3@infradead.org>
Date:   Fri, 6 Apr 2018 14:08:16 -0700
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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@amarulasolutions.com>
>>> Co-Developed-by: Alan Stern <stern@rowland.harvard.edu>
>>> Co-Developed-by: David Howells <dhowells@redhat.com>
>>> Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
>>> Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
>>> Signed-off-by: David Howells <dhowells@redhat.com>
>>> Cc: Will Deacon <will.deacon@arm.com>
>>> Cc: Peter Zijlstra <peterz@infradead.org>
>>> Cc: Boqun Feng <boqun.feng@gmail.com>
>>> Cc: Nicholas Piggin <npiggin@gmail.com>
>>> Cc: Jade Alglave <j.alglave@ucl.ac.uk>
>>> Cc: Luc Maranget <luc.maranget@inria.fr>
>>> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
>>> Cc: Akira Yokosawa <akiyks@gmail.com>
>>> Cc: Ingo Molnar <mingo@kernel.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.

Thanks.

> 							Thanx, Paul
> 
>>> + *
>>> + * Note that the function only tells you that the spinlock is
>>> + * seen to be locked, not that it is locked on your CPU.
>>> + *
>>> + * Further, on CONFIG_SMP=n builds with CONFIG_DEBUG_SPINLOCK=n,
>>> + * the return value is always 0 (see include/linux/spinlock_up.h).
>>> + * Therefore you should not rely heavily on the return value.
>>> + */
>>>  static __always_inline int spin_is_locked(spinlock_t *lock)
>>>  {
>>>  	return raw_spin_is_locked(&lock->rlock);
>>>
>>
>>
>> -- 
>> ~Randy
>>
> 


-- 
~Randy