syslog size unread: was: [PATCH v3 3/3] printk: use the lockless ringbuffer

John Ogness john.ogness at
Mon Jun 29 17:51:40 EDT 2020

On 2020-06-25, Petr Mladek <pmladek at> wrote:
> On Thu 2020-06-18 16:55:19, John Ogness wrote:
>> --- a/kernel/printk/printk.c
>> +++ b/kernel/printk/printk.c
>> @@ -1609,11 +1633,15 @@ int do_syslog(int type, char __user *buf, int len, int source)
>>  		break;
>>  	/* Number of chars in the log buffer */
>> +		if (source != SYSLOG_FROM_PROC) {
>> +			text = kmalloc(LOG_LINE_MAX + PREFIX_MAX, GFP_KERNEL);
>> +			if (!text)
>> +				return -ENOMEM;
> The buffer is needed only to count lines to count the size of added
> prefixes. Could we use the new prb_read_valid_info() that allows to
> get the number of lines without actually reading the buffer?


For the next version I introduce a macro to iterate just the meta data:

#define prb_for_each_info(from, rb, s, i, lc) \
for ((s) = from; prb_read_valid_info(rb, s, i, lc); (s) = (i)->seq + 1)

This can be used in all 3 locations where prb_count_lines() is used. In
all three places, there is no need to be copying the data. And for
SYSLOG_ACTION_SIZE_UNREAD, there is no need for the record and kmalloc'd

This also means that prb_count_lines() will become a private static
helper of the ringbuffer.

John Ogness

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