Re: [PATCH next v2 2/2] printk: fix cpu lock ordering

[Petr Mladek]

On Tue 2021-06-08 16:18:51, John Ogness wrote:
> On 2021-06-08, Petr Mladek <pmladek@xxxxxxxx> wrote:
> > The change makes perfect sense and the code looks correct.
> > But I am not sure about the description of the memory barriers.
> 
> OK.
> 
> >> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> >> index f94babb38493..8c870581cfb4 100644
> >> --- a/kernel/printk/printk.c
> >> +++ b/kernel/printk/printk.c
> >> @@ -3560,10 +3560,29 @@ void printk_cpu_lock_irqsave(bool *lock_flag, unsigned long *irq_flags)
> >>  
> >>  	cpu = smp_processor_id();
> >>  
> >> -	old = atomic_cmpxchg(&printk_cpulock_owner, -1, cpu);
> >> +	/*
> >> +	 * Guarantee loads and stores from the previous lock owner are
> >> +	 * visible to this CPU once it is the lock owner. This pairs
> >> +	 * with cpu_unlock:B.
> >
> > These things are not easy to describe. It took me quite some time to
> > understand the above description. And think that it does not say
> > the full storry.
> >
> > IMHO, the lock should work the way that:
> >
> >    + The new owner see all writes done or seen by the previous owner(s).
> >    + The previous owner(s) never see writes done by the new owner.
> 
> You are right. I can describe those independently.
> 
> > Honestly, I am not sure if we could describe the barriers correctly
> > and effectively at the same time.
> 
> For v3 I would describe the 2 cases separately. For lock/acquire:
> 
> 	/*
> 	 * Guarantee loads and stores from this CPU when it is the lock owner
> 	 * are _not_ visible to the previous lock owner. This pairs with
> 	 * cpu_unlock:B.

Sounds better to me.

*
> 	 * Memory barrier involvement:
> 	 *
> 	 * If cpu_lock:A reads from cpu_unlock:B, then cpu_unlock:A can never
> 	 * read from cpu_lock:B.
> 	 *
> 	 * Relies on:
> 	 *
> 	 * RELEASE from cpu_unlock:A to cpu_unlock:B
> 	 *    matching
> 	 * ACQUIRE from cpu_lock:A to cpu_lock:B
> 	 */

I can't check this so I believe you ;-)

> And for unlock/release:
> 
> 	/*
> 	 * Guarantee loads and stores from this CPU when it was the
> 	 * lock owner are visible to the next lock owner. This pairs
> 	 * with cpu_lock:A.

Sounds good.

*
> 	 * Memory barrier involvement:
> 	 *
> 	 * If cpu_lock:A reads from cpu_unlock:B, then cpu_lock:B
> 	 * reads from cpu_unlock:A.
> 	 *
> 	 * Relies on:
> 	 *
> 	 * RELEASE from cpu_unlock:A to cpu_unlock:B
> 	 *    matching
> 	 * ACQUIRE from cpu_lock:A to cpu_lock:B
> 	 */

Same as for acquire ;-)

> I know you are not a fan of these drawn out memory barrier comments. But
> it really simplifies verification and translation to litmus
> tests. Without such comments, I would be lost looking back at
> printk_ringbuffer.c.
> 
> If the previous dump_stack() cpu lock implementation had such comments,
> we would know if the missing memory barriers were by design.

Sure. I am fine with the comments as long as there is also some human
readable description of the barrier intention.

Best Regards,
Petr