linux-kernel - Re: [PATCH v10 01/21] rcuref: Provide rcuref_is

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Message-ID: <20250313042311.GA1395318@joelnvbox>
Date: Thu, 13 Mar 2025 00:23:11 -0400
From: Joel Fernandes <joelagnelf@...dia.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org,
	André Almeida <andrealmeid@...lia.com>,
	Darren Hart <dvhart@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v10 01/21] rcuref: Provide rcuref_is_dead().

On Wed, Mar 12, 2025 at 04:16:14PM +0100, Sebastian Andrzej Siewior wrote:
> rcuref_read() returns the number of references that are currently held.
> If 0 is returned then it is not safe to assume that the object ca be
> scheduled for deconstruction because it is marked DEAD. This happens if
> the return value of rcuref_put() is ignored and assumptions are made.
> 
> If 0 is returned then the counter transitioned from 0 to RCUREF_NOREF.
> If rcuref_put() did not return to the caller then the counter did not
> yet transition from RCUREF_NOREF to RCUREF_DEAD. This means that there
> is still a chance that the counter counter will transition from
> RCUREF_NOREF to 0 meaning it is still valid and must not be
> deconstructed. In this brief window rcuref_read() will return 0.
> 
> Provide rcuref_is_dead() to determine if the counter is marked as
> RCUREF_DEAD.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  include/linux/rcuref.h | 22 +++++++++++++++++++++-
>  1 file changed, 21 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/rcuref.h b/include/linux/rcuref.h
> index 6322d8c1c6b42..2fb2af6d98249 100644
> --- a/include/linux/rcuref.h
> +++ b/include/linux/rcuref.h
> @@ -30,7 +30,11 @@ static inline void rcuref_init(rcuref_t *ref, unsigned int cnt)
>   * rcuref_read - Read the number of held reference counts of a rcuref
>   * @ref:	Pointer to the reference count
>   *
> - * Return: The number of held references (0 ... N)
> + * Return: The number of held references (0 ... N). The value 0 does not
> + * indicate that it is safe to schedule the object, protected by this reference
> + * counter, for deconstruction.
> + * If you want to know if the reference counter has been marked DEAD (as
> + * signaled by rcuref_put()) please use rcuread_is_dead().
>   */
>  static inline unsigned int rcuref_read(rcuref_t *ref)
>  {
> @@ -40,6 +44,22 @@ static inline unsigned int rcuref_read(rcuref_t *ref)
>  	return c >= RCUREF_RELEASED ? 0 : c + 1;
>  }
>  
> +/**
> + * rcuref_is_dead -	Check if the rcuref has been already marked dead
> + * @ref:		Pointer to the reference count
> + *
> + * Return: True if the object has been marked DEAD. This signals that a previous
> + * invocation of rcuref_put() returned true on this reference counter meaning
> + * the protected object can safely be scheduled for deconstruction.
> + * Otherwise, returns false.
> + */
> +static inline bool rcuref_is_dead(rcuref_t *ref)
> +{
> +	unsigned int c = atomic_read(&ref->refcnt);
> +
> +	return (c >= RCUREF_RELEASED) && (c < RCUREF_NOREF);
> +}
> +
>  extern __must_check bool rcuref_get_slowpath(rcuref_t *ref);
>  

This makes sense to me, another way I guess to determine if it is dead is
actually to do a get() and see if it fails? Though that would be more
expensive and silly.

FWIW for this patch,
Reviewed-by: Joel Fernandes <joelagnelf@...dia.com>

thanks,

 - Joel