[Linux-kernel-mentees] [PATCH v3 3/6] Documentation: RCU: Convert RCU UP systems to reST

Paul E. McKenney paulmck at linux.ibm.com
Tue Jun 25 16:03:54 UTC 2019


On Tue, Jun 25, 2019 at 01:26:24AM -0500, Jiunn Chang wrote:
> RCU UP systems reST markup.
> 
> Signed-off-by: Jiunn Chang <c0d1n61at3 at gmail.com>
> Reviewed-by: Joel Fernandes (Google) <joel at joelfernandes.org>
> ---
>  Documentation/RCU/UP.txt | 37 +++++++++++++++++++++++--------------
>  1 file changed, 23 insertions(+), 14 deletions(-)
> 
> diff --git a/Documentation/RCU/UP.txt b/Documentation/RCU/UP.txt
> index 53bde717017b..67715a47ae89 100644
> --- a/Documentation/RCU/UP.txt
> +++ b/Documentation/RCU/UP.txt
> @@ -1,17 +1,19 @@
> -RCU on Uniprocessor Systems
> +.. _up_doc:
>  
> +RCU on Uniprocessor Systems
> +===========================
>  
>  A common misconception is that, on UP systems, the call_rcu() primitive
>  may immediately invoke its function.  The basis of this misconception
>  is that since there is only one CPU, it should not be necessary to
>  wait for anything else to get done, since there are no other CPUs for
> -anything else to be happening on.  Although this approach will -sort- -of-
> +anything else to be happening on.  Although this approach will *sort of*
>  work a surprising amount of the time, it is a very bad idea in general.
>  This document presents three examples that demonstrate exactly how bad
>  an idea this is.
>  
> -
>  Example 1: softirq Suicide
> +--------------------------
>  
>  Suppose that an RCU-based algorithm scans a linked list containing
>  elements A, B, and C in process context, and can delete elements from
> @@ -28,8 +30,8 @@ your kernel.
>  This same problem can occur if call_rcu() is invoked from a hardware
>  interrupt handler.
>  
> -
>  Example 2: Function-Call Fatality
> +---------------------------------
>  
>  Of course, one could avert the suicide described in the preceding example
>  by having call_rcu() directly invoke its arguments only if it was called
> @@ -46,11 +48,13 @@ its arguments would cause it to fail to make the fundamental guarantee
>  underlying RCU, namely that call_rcu() defers invoking its arguments until
>  all RCU read-side critical sections currently executing have completed.
>  
> -Quick Quiz #1: why is it -not- legal to invoke synchronize_rcu() in
> -	this case?
> +Quick Quiz #1:
> +	Why is it *not* legal to invoke synchronize_rcu() in this case?
>  
> +:ref:`Answers to Quick Quiz <answer_quick_quiz_up>`
>  
>  Example 3: Death by Deadlock
> +----------------------------
>  
>  Suppose that call_rcu() is invoked while holding a lock, and that the
>  callback function must acquire this same lock.  In this case, if
> @@ -76,25 +80,30 @@ there are cases where this can be quite ugly:
>  If call_rcu() directly invokes the callback, painful locking restrictions
>  or API changes would be required.
>  
> -Quick Quiz #2: What locking restriction must RCU callbacks respect?
> +Quick Quiz #2:
> +	What locking restriction must RCU callbacks respect?
>  
> +:ref:`Answers to Quick Quiz <answer_quick_quiz_up>`
>  
>  Summary
> +-------
>  
>  Permitting call_rcu() to immediately invoke its arguments breaks RCU,
>  even on a UP system.  So do not do it!  Even on a UP system, the RCU
> -infrastructure -must- respect grace periods, and -must- invoke callbacks
> +infrastructure *must* respect grace periods, and *must* invoke callbacks
>  from a known environment in which no locks are held.
>  
> -Note that it -is- safe for synchronize_rcu() to return immediately on
> -UP systems, including !PREEMPT SMP builds running on UP systems.
> +Note that it *is* safe for synchronize_rcu() to return immediately on
> +UP systems, including PREEMPT SMP builds running on UP systems.
>  
> -Quick Quiz #3: Why can't synchronize_rcu() return immediately on
> -	UP systems running preemptable RCU?
> +Quick Quiz #3:
> +	Why can't synchronize_rcu() return immediately on UP systems running
> +	preemptable RCU?
>  
> +.. _answer_quick_quiz_up:

As long as you are in the area, the answer is overly constraining.
The locking primitives could use either _irq suffixes (as stated) or
_bh suffixes.  This is obviously not your fault, but please feel free
to fix this with an additional patch.

							Thanx, Paul

>  Answer to Quick Quiz #1:
> -	Why is it -not- legal to invoke synchronize_rcu() in this case?
> +	Why is it *not* legal to invoke synchronize_rcu() in this case?
>  
>  	Because the calling function is scanning an RCU-protected linked
>  	list, and is therefore within an RCU read-side critical section.
> @@ -119,7 +128,7 @@ Answer to Quick Quiz #2:
>  
>  	This restriction might seem gratuitous, since very few RCU
>  	callbacks acquire locks directly.  However, a great many RCU
> -	callbacks do acquire locks -indirectly-, for example, via
> +	callbacks do acquire locks *indirectly*, for example, via
>  	the kfree() primitive.
>  
>  Answer to Quick Quiz #3:
> -- 
> 2.22.0
> 



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