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Message-ID: <YL+DjNG0uhg/uT+C@alley>
Date: Tue, 8 Jun 2021 16:49:48 +0200
From: Petr Mladek <pmladek@...e.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>,
Steven Rostedt <rostedt@...dmis.org>,
Thomas Gleixner <tglx@...utronix.de>,
linux-kernel@...r.kernel.org,
Peter Zijlstra <peterz@...radead.org>,
"Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH next v2 2/2] printk: fix cpu lock ordering
On Tue 2021-06-08 16:18:51, John Ogness wrote:
> On 2021-06-08, Petr Mladek <pmladek@...e.com> 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
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