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

[Petr Mladek <pmladek@suse.com>]

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