[PATCH printk-rework 06/14] printk: use seqcount_latch for clear_seq

John Ogness
2021-02-18T09:40:04Z

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>
---
kernel/printk/printk.c | 58 ++++++++++++++++++++++++++++++++++++------
1 file changed, 50 insertions(+), 8 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f79e7515b5f1..a71e0d41ccb5 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -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)
{
@@ -802,7 +840,7 @@ 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 */
@@ -961,6 +999,9 @@ 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

@@ -1558,7 +1599,8 @@ 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);

@@ -1585,7 +1627,7 @@ 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);
@@ -1595,7 +1637,7 @@ 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();
}

@@ -3332,7 +3374,7 @@ 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);

@@ -3530,7 +3572,7 @@ 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);
}

--
2.20.1