[PATCH printk-rework 08/14] printk: add syslog_lock

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

The global variables @syslog_seq, @syslog_partial, @syslog_time
and write access to @clear_seq are protected by @logbuf_lock.
Once @logbuf_lock is removed, these variables will need their
own synchronization method. Introduce @syslog_lock for this
purpose.

@syslog_lock is a raw_spin_lock for now. This simplifies the
transition to removing @logbuf_lock. Once @logbuf_lock and the
safe buffers are removed, @syslog_lock can change to spin_lock.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
kernel/printk/printk.c | 41 +++++++++++++++++++++++++++++++++++++----
1 file changed, 37 insertions(+), 4 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 20c21a25143d..401df370832b 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -390,8 +390,12 @@ DEFINE_RAW_SPINLOCK(logbuf_lock);
printk_safe_exit_irqrestore(flags); \
} while (0)

+/* syslog_lock protects syslog_* variables and write access to clear_seq. */
+static DEFINE_RAW_SPINLOCK(syslog_lock);
+
#ifdef CONFIG_PRINTK
DECLARE_WAIT_QUEUE_HEAD(log_wait);
+/* All 3 protected by @syslog_lock. */
/* the next printk record to read by syslog(READ) or /proc/kmsg */
static u64 syslog_seq;
static size_t syslog_partial;
@@ -410,7 +414,7 @@ struct latched_seq {
/*
* 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.
+ * access a valid value. Writers are synchronized by @syslog_lock.
*/
static struct latched_seq clear_seq = {
.latch = SEQCNT_LATCH_ZERO(clear_seq.latch),
@@ -470,7 +474,7 @@ bool printk_percpu_data_ready(void)
return __printk_percpu_data_ready;
}

-/* Must be called under logbuf_lock. */
+/* Must be called under syslog_lock. */
static void latched_seq_write(struct latched_seq *ls, u64 val)
{
raw_write_seqcount_latch(&ls->latch);
@@ -1530,7 +1534,9 @@ static int syslog_print(char __user *buf, int size)
size_t skip;

logbuf_lock_irq();
+ raw_spin_lock(&syslog_lock);
if (!prb_read_valid(prb, syslog_seq, &r)) {
+ raw_spin_unlock(&syslog_lock);
logbuf_unlock_irq();
break;
}
@@ -1560,6 +1566,7 @@ static int syslog_print(char __user *buf, int size)
syslog_partial += n;
} else
n = 0;
+ raw_spin_unlock(&syslog_lock);
logbuf_unlock_irq();

if (!n)
@@ -1626,8 +1633,11 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
break;
}

- if (clear)
+ if (clear) {
+ raw_spin_lock(&syslog_lock);
latched_seq_write(&clear_seq, seq);
+ raw_spin_unlock(&syslog_lock);
+ }
logbuf_unlock_irq();

kfree(text);
@@ -1637,10 +1647,24 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
static void syslog_clear(void)
{
logbuf_lock_irq();
+ raw_spin_lock(&syslog_lock);
latched_seq_write(&clear_seq, prb_next_seq(prb));
+ raw_spin_unlock(&syslog_lock);
logbuf_unlock_irq();
}

+/* Return a consistent copy of @syslog_seq. */
+static u64 read_syslog_seq_irq(void)
+{
+ u64 seq;
+
+ raw_spin_lock_irq(&syslog_lock);
+ seq = syslog_seq;
+ raw_spin_unlock_irq(&syslog_lock);
+
+ return seq;
+}
+
int do_syslog(int type, char __user *buf, int len, int source)
{
struct printk_info info;
@@ -1664,8 +1688,9 @@ int do_syslog(int type, char __user *buf, int len, int source)
return 0;
if (!access_ok(buf, len))
return -EFAULT;
+
error = wait_event_interruptible(log_wait,
- prb_read_valid(prb, syslog_seq, NULL));
+ prb_read_valid(prb, read_syslog_seq_irq(), NULL));
if (error)
return error;
error = syslog_print(buf, len);
@@ -1714,8 +1739,10 @@ int do_syslog(int type, char __user *buf, int len, int source)
/* Number of chars in the log buffer */
case SYSLOG_ACTION_SIZE_UNREAD:
logbuf_lock_irq();
+ raw_spin_lock(&syslog_lock);
if (!prb_read_valid_info(prb, syslog_seq, &info, NULL)) {
/* No unread messages. */
+ raw_spin_unlock(&syslog_lock);
logbuf_unlock_irq();
return 0;
}
@@ -1744,6 +1771,7 @@ int do_syslog(int type, char __user *buf, int len, int source)
}
error -= syslog_partial;
}
+ raw_spin_unlock(&syslog_lock);
logbuf_unlock_irq();
break;
/* Size of the log buffer */
@@ -2986,7 +3014,12 @@ void register_console(struct console *newcon)
*/
exclusive_console = newcon;
exclusive_console_stop_seq = console_seq;
+
+ /* Get a consistent copy of @syslog_seq. */
+ raw_spin_lock(&syslog_lock);
console_seq = syslog_seq;
+ raw_spin_unlock(&syslog_lock);
+
logbuf_unlock_irqrestore(flags);
}
console_unlock();
--
2.20.1