Re: [PATCH printk-rework 08/14] printk: add syslog_lock

[Helge Deller]

On 2/19/21 5:33 PM, John Ogness wrote:
> Added CC: linux-parisc@xxxxxxxxxxxxxxx
>
> On 2021-02-19, John Ogness <john.ogness@xxxxxxxxxxxxx> wrote:
> > > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > > > index 20c21a25143d..401df370832b 100644
> > > > --- a/kernel/printk/printk.c
> > > > +++ b/kernel/printk/printk.c
> > > > +/* Return a consistent copy of @syslog_seq. */
> > > > +static u64 read_syslog_seq_irq(void)
> > > > +{
> > > > +	u64 seq;
> > > > +
> > > > +	raw_spin_lock_irq(&syslog_lock);
> > > > +	seq = syslog_seq;
> > > > +	raw_spin_unlock_irq(&syslog_lock);
> > >
> > > Is there any particular reason to disable interrupts here?
> > >
> > > It would make sense only when the lock could be taken in IRQ
> > > context. Then we would need to always disable interrupts when
> > > the lock is taken. And if it is taken in IRQ context, we would
> > > need to safe flags.
> >
> > All other instances of locking @syslog_lock are done with interrupts
> > disabled. And we have:
> >
> > register_console()
> >    logbuf_lock_irqsave()
> >      raw_spin_lock(&syslog_lock)
> >
> > I suppose I need to go through all the console drivers to see if any
> > register in interrupt context. If not, that logbuf_lock_irqsave()
> > should be replaced with logbuf_lock_irq(). And then locking
> > @syslog_lock will not need to disable interrupts.
>
> I found a possible call chain in interrupt context. From arch/parisc
> there is the interrupt handler:
>
> handle_interruption(code=1) /* High-priority machine check (HPMC) */
>    pdc_console_restart()
>      pdc_console_init_force()
>        register_console()
>
> All other register_console() calls in the kernel are either during init
> (within __init sections and probe functions) or are clearly not in
> interrupt context (using mutex, kzalloc, spin_lock_irq, etc).
>
> I am not familiar with parisc, but I am assuming handle_interruption()
> is always called with interrupts disabled (unless the HPMC interrupt is
> somehow an exception).

Yes, handle_interruption() is the irq handler, running with irqs off.
HPMC is the crash handler - it's called when the kernel will stop
anyway. pdc_console is a very basic firmware console which prints
the last messages before the machine halts on fatal errors.
So, this code it's not the typical use case....

Helge