On Tue 2021-03-30 17:35:11, John Ogness wrote:
> @syslog_lock was a raw_spin_lock to simplify the transition of
> removing @logbuf_lock and the safe buffers. With that transition
> complete, and since all uses of @syslog_lock are within sleepable
> contexts, @syslog_lock can become a mutex.
It makes perfect sense.
> ---
> Note: The removal of read_syslog_seq_irq() is technically a small
> step backwards. But the follow-up patch moves forward again
> and closes a window that existed with read_syslog_seq_irq()
> and @syslog_lock as a spin_lock.
This change would deserve a comment in the commit message. Well, I do
not think that is a step backward, see below.
> kernel/printk/printk.c | 49 +++++++++++++++++-------------------------
> 1 file changed, 20 insertions(+), 29 deletions(-)
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index f090d6a1b39e..b771aae46445 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1603,21 +1603,9 @@ static int syslog_print_all(char __user *buf, int
> size, bool clear)
>
> static void syslog_clear(void)
> {
> - raw_spin_lock_irq(&syslog_lock);
> + mutex_lock(&syslog_lock);
> latched_seq_write(&clear_seq, prb_next_seq(prb));
> - raw_spin_unlock_irq(&syslog_lock);
> -}
> -
> -/* 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;
> + mutex_unlock(&syslog_lock);
> }
>
> int do_syslog(int type, char __user *buf, int len, int source)
> @@ -1644,8 +1633,12 @@ int do_syslog(int type, char __user *buf, int len, int
> source)
> if (!access_ok(buf, len))
> return -EFAULT;
>
> - error = wait_event_interruptible(log_wait,
> - prb_read_valid(prb, read_syslog_seq_irq(),
> NULL));
> + /* Get a consistent copy of @syslog_seq. */
> + mutex_lock(&syslog_lock);
> + seq = syslog_seq;
> + mutex_unlock(&syslog_lock);
> +
> + error = wait_event_interruptible(log_wait, prb_read_valid(prb,
> seq, NULL));
Honestly, I am not sure how the syslog interface should work when there
are two readers in the system. They both share the same "syslog_seq".
This either fixes a historic bug. The caller of SYSLOG_ACTION_READ
might miss the new message when another reader did read it in the
meantime.
Or it might introduce a regression when two readers would read the
same message.
Or it does not matter because the behavior is racy by definition.
Best Regards,
Petr
PS: I am going to look at this more with a fresh head after Easter
holidays. The answer is important also for the next patch that
basically restores the original behavior.
