On 2/22/21 5:28 PM, Petr Mladek wrote:
On Sun 2021-02-21 22:39:42, Helge Deller wrote:
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....
Thanks for information.
Is this code supposed to work only during early boot or anytime,
please?
No.
It's only called when the kernel completely crashes, when all
spinlocks should get busted and so on.
It's the emergency way to get some info out at least.
Note that it is not safe because register_console() takes
console_lock() which is a sleeping lock.
As I said, in that stage the plan is to bust all spinlocks.
That said, we are going to rework the console handling a lot. We are
trying to remove as many locks from the printk path as possible.
That's good!
I guess that the list of consoles will be synchronized using
rcu at the end. But it is still a long way to go.
I'd say, that you simply should ignore this specific case here.
I'm happy to change anything there, so if you get rid of printk locks
it will benefit here too.
Helge
