x86/dumpstack: Show registers dump with trace's log level

show_trace_log_lvl() provides x86 platform-specific way to unwind
backtrace with a given log level. Unfortunately, registers dump(s) are
not printed with the same log level - instead, KERN_DEFAULT is always
used.

Arista's switches uses quite common setup with rsyslog, where only
urgent messages goes to console (console_log_level=KERN_ERR), everything
else goes into /var/log/ as the console baud-rate often is indecently
slow (9600 bps).

Backtrace dumps without registers printed have proven to be as useful as
morning standups. Furthermore, in order to introduce KERN_UNSUPPRESSED
(which I believe is still the most elegant way to fix raciness of sysrq[1])
the log level should be passed down the stack to register dumping
functions. Besides, there is a potential use-case for printing traces
with KERN_DEBUG level [2] (where registers dump shouldn't appear with
higher log level).

After all preparations are done, provide log_lvl parameter for
show_regs_if_on_stack() and wire up to actual log level used as
an argument for show_trace_log_lvl().

[1]: https://lore.kernel.org/lkml/20190528002412.1625-1-dima@arista.com/
[2]: https://lore.kernel.org/linux-doc/20190724170249.9644-1-dima@arista.com/Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Petr Mladek <pmladek@suse.com>
Link: https://lkml.kernel.org/r/20200629144847.492794-4-dima@arista.com

arch/x86/kernel/dumpstack.c

@@ -134,7 +134,7 @@ void show_iret_regs(struct pt_regs *regs, const char *log_lvl)
 }
 
 static void show_regs_if_on_stack(struct stack_info *info, struct pt_regs *regs,
-				  bool partial)
+				  bool partial, const char *log_lvl)
 {
 	/*
 	 * These on_stack() checks aren't strictly necessary: the unwind code
@@ -146,7 +146,7 @@ static void show_regs_if_on_stack(struct stack_info *info, struct pt_regs *regs,
 	 * they can be printed in the right context.
 	 */
 	if (!partial && on_stack(info, regs, sizeof(*regs))) {
-		__show_regs(regs, SHOW_REGS_SHORT, KERN_DEFAULT);
+		__show_regs(regs, SHOW_REGS_SHORT, log_lvl);
 
 	} else if (partial && on_stack(info, (void *)regs + IRET_FRAME_OFFSET,
 				       IRET_FRAME_SIZE)) {
@@ -155,7 +155,7 @@ static void show_regs_if_on_stack(struct stack_info *info, struct pt_regs *regs,
 		 * full pt_regs might not have been saved yet.  In that case
 		 * just print the iret frame.
 		 */
-		show_iret_regs(regs, KERN_DEFAULT);
+		show_iret_regs(regs, log_lvl);
 	}
 }
 
@@ -210,7 +210,7 @@ void show_trace_log_lvl(struct task_struct *task, struct pt_regs *regs,
 			printk("%s <%s>\n", log_lvl, stack_name);
 
 		if (regs)
-			show_regs_if_on_stack(&stack_info, regs, partial);
+			show_regs_if_on_stack(&stack_info, regs, partial, log_lvl);
 
 		/*
 		 * Scan the stack, printing any text addresses we find.  At the
@@ -271,7 +271,7 @@ void show_trace_log_lvl(struct task_struct *task, struct pt_regs *regs,
 			/* if the frame has entry regs, print them */
 			regs = unwind_get_entry_regs(&state, &partial);
 			if (regs)
-				show_regs_if_on_stack(&stack_info, regs, partial);
+				show_regs_if_on_stack(&stack_info, regs, partial, log_lvl);
 		}
 
 		if (stack_name)