From: John Ogness <john.ogness@linutronix.de>
To: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>,
Steven Rostedt <rostedt@goodmis.org>,
Thomas Gleixner <tglx@linutronix.de>,
linux-kernel@vger.kernel.org,
Peter Zijlstra <peterz@infradead.org>,
"Paul E. McKenney" <paulmck@kernel.org>
Subject: Re: [PATCH next v2 2/2] printk: fix cpu lock ordering
Date: Tue, 08 Jun 2021 16:18:51 +0200 [thread overview]
Message-ID: <875yyoigms.fsf@jogness.linutronix.de> (raw)
In-Reply-To: <YL9osWvgvdCo4JAK@alley>
On 2021-06-08, Petr Mladek <pmladek@suse.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.
*
* 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
*/
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.
*
* 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
*/
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.
John Ogness
next prev parent reply other threads:[~2021-06-08 14:18 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-07 20:02 [PATCH next v2 0/2] introduce printk cpu lock John Ogness
2021-06-07 20:02 ` [PATCH next v2 1/2] dump_stack: move cpu lock to printk.c John Ogness
2021-06-08 2:43 ` kernel test robot
2021-06-08 13:48 ` Petr Mladek
2021-06-10 13:26 ` John Ogness
2021-06-11 7:00 ` Petr Mladek
2021-06-08 11:40 ` Petr Mladek
2021-06-08 13:55 ` John Ogness
2021-06-08 14:54 ` Petr Mladek
2021-06-07 20:02 ` [PATCH next v2 2/2] printk: fix cpu lock ordering John Ogness
2021-06-08 12:55 ` Petr Mladek
2021-06-08 14:18 ` John Ogness [this message]
2021-06-08 14:49 ` Petr Mladek
2021-06-10 14:44 ` John Ogness
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