printk: add syslog_lock

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>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210303101528.29901-11-john.ogness@linutronix.de

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 65e216c..e794a08 100644 (file)
--- 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);
@@ -1529,7 +1533,9 @@ 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;
                }
@@ -1559,6 +1565,7 @@ 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)
@@ -1625,8 +1632,11 @@ 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);
@@ -1636,10 +1646,24 @@ 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;
@@ -1663,8 +1687,9 @@ 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);
@@ -1713,8 +1738,10 @@ 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;
                }
@@ -1743,6 +1770,7 @@ 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 */
@@ -2992,7 +3020,12 @@ 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();