Re: [PATCH printk-rework 08/14] printk: add syslog_lock
From: Helge Deller
Date: Tue Feb 23 2021 - 09:48:31 EST
On 2/23/21 3:23 PM, Petr Mladek wrote:
On Tue 2021-02-23 13:22:22, Helge Deller wrote:
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?
I found a possible call chain in interrupt context. From arch/parisc
there is the interrupt handler:
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.
OK.
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.
Just to be sure. Note that that register_console() does not bust
console_lock in panic.
Ok.
bust_spinlocks() just increments oops_in_progress counter. It has
effect only when the caller checks this variable and use trylock
when it is set. For example, see serial8250_console_write():
void serial8250_console_write(struct uart_8250_port *up, const char *s,
unsigned int count)
{
int locked = 1;
if (oops_in_progress)
locked = spin_trylock_irqsave(&port->lock, flags);
else
spin_lock_irqsave(&port->lock, flags);
...
if (locked)
spin_unlock_irqrestore(&port->lock, flags);
}
register_console() does not check oops_in_progress at the moment
and might get blocked on console_sem.
We could add the checks for oops_in_progress into register_console().
But I am not sure if it is worth it.
It's not worth it just because of parisc.
I haven't seen any such crash in years, so the current implementation
is probably untested and outdated.
It seems that you used this code for ages. The risk of the deadlock
is small. It likely works most of the time. The upcoming printk rework
should allow a cleaner solution.
Yes, it would be great if you can include such a "hard-panic/crash-dump-case"
in the rework.
Helge