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

John Ogness
2021-03-02T13:20:51Z

On 2021-03-01, Petr Mladek <pmladek@suse.com> wrote:
>> 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)

For this caller (arch/powerpc/kernel/nvram_64.c) and for
(kernel/debug/kdb/kdb_main.c), kmsg_dump_rewind() is called twice within
the dumper. So rewind will still be used there.

> 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 suppose we need to add this part anyway, if we want to enforce that
records before @clear_seq are not to be available for dumpers.

> 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?

On a technical level, it does not make any difference. It is pure
cosmetic.

Personally, I prefer the rewind directly before the kmsg_dump_get calls
because it puts the initializer directly next to the user.

As an example to illustrate my view, I prefer:

for (i = 0; i < n; i++)
...;

instead of:

int i = 0;

...

for (; i < n; i++)
...;

Also, I do not really like the special use of 0/U64_MAX to identify
special actions of the kmsg_dump_get functions.

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

Since you do not resist, I will keep the API the same for v4. But I will
add the @clear_seq check to the kmsg_dump_get functions.

John Ogness

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