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Subject: Re: [PATCH v4 1/3] locking: Document the semantics of
 spin_is_locked()
To:     Andrea Parri <andrea.parri@amarulasolutions.com>,
        paulmck@linux.vnet.ibm.com
Cc:     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>
From:   Randy Dunlap <rdunlap@infradead.org>
Message-ID: <b0dbf383-6403-a408-047d-0dd0d2135a03@infradead.org>
Date:   Fri, 6 Apr 2018 14:01:41 -0700
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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.]

> + *
> + * 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