Re: [PATCH next v3 01/15] um: synchronize kmsg_dumper

Petr Mladek
2021-03-01T16:16:38Z

On Thu 2021-02-25 21:24:24, John Ogness wrote:
> The kmsg_dumper can be called from any context and CPU, possibly
> from multiple CPUs simultaneously. Since a static buffer is used
> to retrieve the kernel logs, this buffer must be protected against
> simultaneous dumping. Skip dumping if another context is already
> dumping.
>
> Signed-off-by: John Ogness <john.ogness@linutronix.de>
> Reviewed-by: Petr Mladek <pmladek@suse.com>
> ---
> arch/um/kernel/kmsg_dump.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/arch/um/kernel/kmsg_dump.c b/arch/um/kernel/kmsg_dump.c
> index 6516ef1f8274..4869e2cc787c 100644
> --- a/arch/um/kernel/kmsg_dump.c
> +++ b/arch/um/kernel/kmsg_dump.c
> @@ -1,5 +1,6 @@
> // SPDX-License-Identifier: GPL-2.0
> #include <linux/kmsg_dump.h>
> +#include <linux/spinlock.h>
> #include <linux/console.h>
> #include <linux/string.h>
> #include <shared/init.h>
> @@ -9,6 +10,7 @@
> static void kmsg_dumper_stdout(struct kmsg_dumper *dumper,
> enum kmsg_dump_reason reason)
> {
> + static DEFINE_SPINLOCK(lock);
> static char line[1024];
> struct console *con;
> size_t len = 0;
> @@ -29,11 +31,16 @@ static void kmsg_dumper_stdout(struct kmsg_dumper *dumper,
> if (con)
> return;
>
> + if (!spin_trylock(&lock))

I have almost missed this. It is wrong. The last version correctly
used

if (!spin_trylock_irqsave(&lock, flags))

kmsg_dump(KMSG_DUMP_PANIC) is called in panic() with interrupts
disabled. We have to store the flags here.

Best Regards,
Petr