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printk: use seqcount_latch for clear_seq

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kmsg_dump_rewind_nolock() locklessly reads @clear_seq. However,
this is not done atomically. Since @clear_seq is 64-bit, this
cannot be an atomic operation for all platforms. Therefore, use
a seqcount_latch to allow readers to always read a consistent
value.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210303101528.29901-9-john.ogness@linutronix.de
This commit is contained in:
John Ogness 2021-03-03 11:15:21 +01:00 committed by Petr Mladek
parent cf5b0208fd
commit 7d7a23a91c
1 changed files with 50 additions and 8 deletions
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58
kernel/printk/printk.c
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@ -402,8 +402,21 @@ static u64 console_seq;
static u64 exclusive_console_stop_seq;
static unsigned long console_dropped;
/* the next printk record to read after the last 'clear' command */
static u64 clear_seq;
struct latched_seq {
seqcount_latch_t latch;
u64 val[2];
};
/*
* The next printk record to read after the last 'clear' command. There are
* two copies (updated with seqcount_latch) so that reads can locklessly
* access a valid value. Writers are synchronized by @logbuf_lock.
*/
static struct latched_seq clear_seq = {
.latch = SEQCNT_LATCH_ZERO(clear_seq.latch),
.val[0] = 0,
.val[1] = 0,
};
#ifdef CONFIG_PRINTK_CALLER
#define PREFIX_MAX 48
@ -457,6 +470,31 @@ bool printk_percpu_data_ready(void)
return __printk_percpu_data_ready;
}
/* Must be called under logbuf_lock. */
static void latched_seq_write(struct latched_seq *ls, u64 val)
{
raw_write_seqcount_latch(&ls->latch);
ls->val[0] = val;
raw_write_seqcount_latch(&ls->latch);
ls->val[1] = val;
}
/* Can be called from any context. */
static u64 latched_seq_read_nolock(struct latched_seq *ls)
{
unsigned int seq;
unsigned int idx;
u64 val;
do {
seq = raw_read_seqcount_latch(&ls->latch);
idx = seq & 0x1;
val = ls->val[idx];
} while (read_seqcount_latch_retry(&ls->latch, seq));
return val;
}
/* Return log buffer address */
char *log_buf_addr_get(void)
{
@ -801,7 +839,7 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
* like issued by 'dmesg -c'. Reading /dev/kmsg itself
* changes no global state, and does not clear anything.
*/
user->seq = clear_seq;
user->seq = latched_seq_read_nolock(&clear_seq);
break;
case SEEK_END:
/* after the last record */
@ -960,6 +998,9 @@ void log_buf_vmcoreinfo_setup(void)
VMCOREINFO_SIZE(atomic_long_t);
VMCOREINFO_TYPE_OFFSET(atomic_long_t, counter);
VMCOREINFO_STRUCT_SIZE(latched_seq);
VMCOREINFO_OFFSET(latched_seq, val);
}
#endif
@ -1557,7 +1598,8 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
* Find first record that fits, including all following records,
* into the user-provided buffer for this dump.
*/
seq = find_first_fitting_seq(clear_seq, -1, size, true, time);
seq = find_first_fitting_seq(latched_seq_read_nolock(&clear_seq), -1,
size, true, time);
prb_rec_init_rd(&r, &info, text, CONSOLE_LOG_MAX);
@ -1584,7 +1626,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
}
if (clear)
clear_seq = seq;
latched_seq_write(&clear_seq, seq);
logbuf_unlock_irq();
kfree(text);
@ -1594,7 +1636,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
static void syslog_clear(void)
{
logbuf_lock_irq();
clear_seq = prb_next_seq(prb);
latched_seq_write(&clear_seq, prb_next_seq(prb));
logbuf_unlock_irq();
}
@ -3336,7 +3378,7 @@ void kmsg_dump(enum kmsg_dump_reason reason)
dumper->active = true;
logbuf_lock_irqsave(flags);
dumper->cur_seq = clear_seq;
dumper->cur_seq = latched_seq_read_nolock(&clear_seq);
dumper->next_seq = prb_next_seq(prb);
logbuf_unlock_irqrestore(flags);
@ -3534,7 +3576,7 @@ EXPORT_SYMBOL_GPL(kmsg_dump_get_buffer);
*/
void kmsg_dump_rewind_nolock(struct kmsg_dumper *dumper)
{
dumper->cur_seq = clear_seq;
dumper->cur_seq = latched_seq_read_nolock(&clear_seq);
dumper->next_seq = prb_next_seq(prb);
}