linux-kernel - Re: [PATCH printk v2 4/5] printk: convert @syslog

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Date:   Thu, 1 Apr 2021 17:13:46 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk v2 4/5] printk: convert @syslog_lock to mutex

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.