Re: [PATCH next v3 12/15] printk: introduce a kmsg_dump iterator

Petr Mladek
2021-03-01T18:07:41Z

On Thu 2021-02-25 21:24:35, John Ogness wrote:
> Rather than storing the iterator information in the registered
> kmsg_dumper structure, create a separate iterator structure. The
> kmsg_dump_iter structure can reside on the stack of the caller, thus
> allowing lockless use of the kmsg_dump functions.
>
> This change also means that the kmsg_dumper dump() callback no
> longer needs to pass in the kmsg_dumper as an argument. If
> kmsg_dumpers want to access the kernel logs, they can use the new
> iterator.
>
> Update the kmsg_dumper callback prototype. Update code that accesses
> the kernel logs using the kmsg_dumper structure to use the new
> kmsg_dump_iter structure. For kmsg_dumpers, this also means adding a
> call to kmsg_dump_rewind() to initialize the iterator.
>
> All this is in preparation for removal of @logbuf_lock.
>
> Signed-off-by: John Ogness <john.ogness@linutronix.de>
> ---
> arch/powerpc/kernel/nvram_64.c | 14 +++---
> arch/powerpc/platforms/powernv/opal-kmsg.c | 3 +-
> arch/powerpc/xmon/xmon.c | 6 +--
> arch/um/kernel/kmsg_dump.c | 8 +--
> drivers/hv/vmbus_drv.c | 7 +--
> drivers/mtd/mtdoops.c | 8 +--
> fs/pstore/platform.c | 8 +--
> include/linux/kmsg_dump.h | 38 ++++++++-------
> kernel/debug/kdb/kdb_main.c | 10 ++--
> kernel/printk/printk.c | 57 ++++++++++------------
> 10 files changed, 81 insertions(+), 78 deletions(-)
>
> diff --git a/arch/powerpc/kernel/nvram_64.c b/arch/powerpc/kernel/nvram_64.c
> index 532f22637783..5a64b24a91c2 100644
> --- a/arch/powerpc/kernel/nvram_64.c
> +++ b/arch/powerpc/kernel/nvram_64.c
> @@ -72,8 +72,7 @@ static const char *nvram_os_partitions[] = {
> NULL
> };
>
> -static void oops_to_nvram(struct kmsg_dumper *dumper,
> - enum kmsg_dump_reason reason);
> +static void oops_to_nvram(enum kmsg_dump_reason reason);
>
> static struct kmsg_dumper nvram_kmsg_dumper = {
> .dump = oops_to_nvram
> @@ -642,11 +641,11 @@ void __init nvram_init_oops_partition(int rtas_partition_exists)
> * that we think will compress sufficiently to fit in the lnx,oops-log
> * partition. If that's too much, go back and capture uncompressed text.
> */
> -static void oops_to_nvram(struct kmsg_dumper *dumper,
> - enum kmsg_dump_reason reason)
> +static void oops_to_nvram(enum kmsg_dump_reason reason)
> {
> struct oops_log_info *oops_hdr = (struct oops_log_info *)oops_buf;
> static unsigned int oops_count = 0;
> + static struct kmsg_dump_iter iter;
> static bool panicking = false;
> static DEFINE_SPINLOCK(lock);
> unsigned long flags;
> @@ -681,13 +680,14 @@ static void oops_to_nvram(struct kmsg_dumper *dumper,
> return;
>
> if (big_oops_buf) {
> - kmsg_dump_get_buffer(dumper, false,
> + kmsg_dump_rewind(&iter);

It would be nice to get rid of the kmsg_dump_rewind(&iter) calls
in all callers.

A solution might be to create the following in include/linux/kmsg_dump.h

#define KMSG_DUMP_ITER_INIT(iter) { \
.cur_seq = 0, \
.next_seq = U64_MAX, \
}

#define DEFINE_KMSG_DUMP_ITER(iter) \
struct kmsg_dump_iter iter = KMSG_DUMP_ITER_INIT(iter)

Then we could do the following at the beginning of both
kmsg_dump_get_buffer() and kmsg_dump_get_line():

u64 clear_seq = latched_seq_read_nolock(&clear_seq);

if (iter->cur_seq < clear_seq)
cur_seq = clear_seq;

I am not completely sure about next_seq:

+ kmsg_dump_get_buffer() will set it for the next call anyway.
It reads the blocks of messages from the newest.

+ kmsg_dump_get_line() wants to read the entire buffer anyway.
But there is a small risk of an infinite loop when new messages
are printed when dumping each line.

It might be better to avoid the infinite loop. We could do the following:

static void check_and_set_iter(struct kmsg_dump_iter)
{
if (iter->cur_seq == 0 && iter->next_seq == U64_MAX) {
kmsg_dump_rewind(iter);
}

and call this at the beginning of both kmsg_dump_get_buffer()
and kmsg_dump_get_line()

What do you think?

Note that I do not resist on it. But it might make the API easier to
use from my POV.

Otherwise the patch looks good to me.

Best Regards,
Petr