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

Petr Mladek
2024-02-07T16:53:26Z

On Wed 2024-02-07 17:18:38, Petr Mladek wrote:
> 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.

JFYI, it has been committed into printk/linux.git,
branch rework/console-flushing-fixes.

Let's keep our fingers crossed.

Best Regards,
Petr