Re: [PATCH printk v4 20/27] printk: Avoid console_lock dance if no legacy or boot consoles

[John Ogness <john.ogness@linutronix.de>]

On 2024-04-03, John Ogness <john.ogness@linutronix.de> wrote:
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index df84c6bfbb2d..4ff3800e8e8e 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3810,6 +3833,7 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
>  	u64 last_diff = 0;
>  	u64 printk_seq;
>  	short flags;
> +	bool locked;
>  	int cookie;
>  	u64 diff;
>  	u64 seq;
> @@ -3819,22 +3843,35 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
>  	seq = prb_next_reserve_seq(prb);
>  
>  	/* Flush the consoles so that records up to @seq are printed. */
> -	console_lock();
> -	console_unlock();
> +	if (printing_via_unlock) {
> +		console_lock();
> +		console_unlock();
> +	}
>  
>  	for (;;) {
>  		unsigned long begin_jiffies;
>  		unsigned long slept_jiffies;
>  
> +		locked = false;
>  		diff = 0;
>  
> +		if (printing_via_unlock) {
> +			/*
> +			 * Hold the console_lock to guarantee safe access to
> +			 * console->seq. Releasing console_lock flushes more
> +			 * records in case @seq is still not printed on all
> +			 * usable consoles.
> +			 */
> +			console_lock();
> +			locked = true;
> +		}
> +
>  		/*
> -		 * Hold the console_lock to guarantee safe access to
> -		 * console->seq. Releasing console_lock flushes more
> -		 * records in case @seq is still not printed on all
> -		 * usable consoles.
> +		 * Ensure the compiler does not optimize @locked to be
> +		 * @printing_via_unlock since the latter can change at any
> +		 * time.
>  		 */
> -		console_lock();
> +		barrier();

When I originally implemented this, my objective was to force the
compiler to use a local variable. But to be fully paranoid (which we
should) we must also forbid the compiler from being able to do this
nonsense:

if (printing_via_unlock) {
	console_lock();
	locked = printing_via_unlock;
}

I suggest replacing the __pr_flush() hunks to be as follows instead
(i.e. ensure all conditional console lock usage within the loop is using
the local variable):

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index df84c6bfbb2d..1dbd2a837b67 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3819,22 +3842,34 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
 	seq = prb_next_reserve_seq(prb);
 
 	/* Flush the consoles so that records up to @seq are printed. */
-	console_lock();
-	console_unlock();
+	if (printing_via_unlock) {
+		console_lock();
+		console_unlock();
+	}
 
 	for (;;) {
 		unsigned long begin_jiffies;
 		unsigned long slept_jiffies;
-
-		diff = 0;
+		bool use_console_lock = printing_via_unlock;
 
 		/*
-		 * Hold the console_lock to guarantee safe access to
-		 * console->seq. Releasing console_lock flushes more
-		 * records in case @seq is still not printed on all
-		 * usable consoles.
+		 * Ensure the compiler does not optimize @use_console_lock to
+		 * be @printing_via_unlock since the latter can change at any
+		 * time.
 		 */
-		console_lock();
+		barrier();
+
+		diff = 0;
+
+		if (use_console_lock) {
+			/*
+			 * Hold the console_lock to guarantee safe access to
+			 * console->seq. Releasing console_lock flushes more
+			 * records in case @seq is still not printed on all
+			 * usable consoles.
+			 */
+			console_lock();
+		}
 
 		cookie = console_srcu_read_lock();
 		for_each_console_srcu(c) {
@@ -3854,6 +3889,7 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
 			if (flags & CON_NBCON) {
 				printk_seq = nbcon_seq_read(c);
 			} else {
+				WARN_ON_ONCE(!use_console_lock);
 				printk_seq = c->seq;
 			}
 
@@ -3865,7 +3901,8 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
 		if (diff != last_diff && reset_on_progress)
 			remaining_jiffies = timeout_jiffies;
 
-		console_unlock();
+		if (use_console_lock)
+			console_unlock();
 
 		/* Note: @diff is 0 if there are no usable consoles. */
 		if (diff == 0 || remaining_jiffies == 0)