On Tue 2021-01-26 22:21:48, John Ogness wrote:
> In preparation for removing printk's @logbuf_lock, dumpers that have
> assumed to be protected against parallel calls must provide their own
> synchronization. Add a locally static spinlock to synchronize the
> kmsg_dump call and temporary buffer usage.
>
> Signed-off-by: John Ogness <[email protected]>
> ---
> arch/um/kernel/kmsg_dump.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/arch/um/kernel/kmsg_dump.c b/arch/um/kernel/kmsg_dump.c
> index f38349ad00ea..173999422ed8 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 <shared/init.h>
> #include <shared/kern.h>
> @@ -9,8 +10,10 @@ static void kmsg_dumper_stdout(struct kmsg_dumper *dumper,
> enum kmsg_dump_reason reason,
> struct kmsg_dumper_iter *iter)
> {
> + static DEFINE_SPINLOCK(lock);
> static char line[1024];
> struct console *con;
> + unsigned long flags;
> size_t len = 0;
>
> /* only dump kmsg when no console is available */
> @@ -25,11 +28,16 @@ static void kmsg_dumper_stdout(struct kmsg_dumper *dumper,
> if (con)
> return;
>
> + if (!spin_trylock_irqsave(&lock, flags))
> + return;
> +
> printf("kmsg_dump:\n");
> while (kmsg_dump_get_line(iter, true, line, sizeof(line), &len)) {
> line[len] = '\0';
> printf("%s", line);
> }
> +
> + spin_unlock_irqrestore(&lock, flags);
What exactly is synchronized here, please?
Access to @line buffer or @iter or both?
It looks to me that the access to @line buffer was not synchronized
before. kmsg_dump_get_line() used a lock internally but
it was not synchronized with the later printf("%s", line);
The only remaining reason might be the access to @iter. But it is on stack
in kmsg_dump(). Nobody else could have access to it.
IMHO, this patch is not needed.
Do I miss something, please?
Best Regards,
Petr
