linux-kernel - Re: [PATCH printk v4 00/14] fix console flushing

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Date: Wed, 7 Feb 2024 17:18:36 +0100
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,
	Mukesh Ojha <quic_mojha@...cinc.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Kefeng Wang <wangkefeng.wang@...wei.com>,
	Arnd Bergmann <arnd@...db.de>, Uros Bizjak <ubizjak@...il.com>,
	"Guilherme G. Piccoli" <gpiccoli@...lia.com>
Subject: Re: [PATCH printk v4 00/14] fix console flushing

On Wed 2024-02-07 14:46:49, John Ogness wrote:
> Hi,
> 
> While testing various flushing scenarios, I stumbled on a few
> issues that cause console flushing to fail. While at LPC2023 in
> Richmond, I sat down with Petr Mladek and we reviewed the
> v2 [0] series. This series is the result of that offline
> discussion. v3 is here [1].
> 
> This series addresses the following issues:
> 
> 1. The prb_next_seq() optimization caused inconsistent return
>    values. Fix prb_next_seq() to the originally intended
>    behavior but keep an optimization.
> 
> 2. pr_flush() might not wait until the most recently stored
>    printk() message if non-finalized records precede it. Fix
>    pr_flush() to wait for all records to print that are at
>    least reserved at the time of the call.
> 
> 3. In panic, the panic messages will not print if non-finalized
>    records precede them. Add a special condition so that
>    readers on the panic CPU will drop records that are not in
>    a consistent state.
> 
> 4. It is possible (and easy to reproduce) a scenario where the
>    console on the panic CPU hands over to a waiter of a stopped
>    CPU. Do not use the handover feature in panic.
> 
> 5. If messages are being dropped during panic, non-panic CPUs
>    are silenced. But by then it is already too late and most
>    likely the panic messages have been dropped. Change the
>    non-panic CPU silencing logic to _immediately_ silence
>    non-panic CPUs during panic. This also leads to clean panic
>    output when many CPUs are blasting the kernel log.
> 
> 6. If a panic occurs in a context where printk() calls defer
>    printing (NMI or printk_safe section), the printing of the
>    final panic messages rely on irq_work. If that mechanism is
>    not available, the final panic messages are not seen (even
>    though they are finalized in the ringbuffer). Add one last
>    explicit flush after all printk() calls are finished to
>    ensure all available messages in the kernel log are printed.
> 
> 7. When dumping the stacktrace from panic(), do not use the
>    printk_cpu_sync because it can deadlock if another CPU holds
>    and is unable to release the printk_cpu_sync.
> 
> This series also performs some minor cleanups to remove open
> coded checks about the panic context and improve documentation
> language regarding data-less records.
> 
> Because of multiple refactoring done in recent history, it
> would be helpful to provide the LTS maintainers with the proper
> backported patches. I am happy to do this.

The series seems to be ready linux-next from my POV. I am going
to push it there so that it gets as much testing before
the merge window as possible.

Best Regards,
Petr