Patch "dump_stack: Do not get cpu_sync for panic CPU" has been added to the 6.8-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    dump_stack: Do not get cpu_sync for panic CPU

to the 6.8-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     dump_stack-do-not-get-cpu_sync-for-panic-cpu.patch
and it can be found in the queue-6.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit ffdb28902359d97bbf24eaa25d7b1585a0f91142
Author: John Ogness <john.ogness@xxxxxxxxxxxxx>
Date:   Wed Feb 7 14:47:03 2024 +0106

    dump_stack: Do not get cpu_sync for panic CPU
    
    [ Upstream commit 7412dc6d55eed6b76180e40ac3601412ebde29bd ]
    
    dump_stack() is called in panic(). If for some reason another CPU
    is holding the printk_cpu_sync and is unable to release it, the
    panic CPU will be unable to continue and print the stacktrace.
    
    Since non-panic CPUs are not allowed to store new printk messages
    anyway, there is no need to synchronize the stacktrace output in
    a panic situation.
    
    For the panic CPU, do not get the printk_cpu_sync because it is
    not needed and avoids a potential deadlock scenario in panic().
    
    Link: https://lore.kernel.org/lkml/ZcIGKU8sxti38Kok@alley
    Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
    Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
    Link: https://lore.kernel.org/r/20240207134103.1357162-15-john.ogness@xxxxxxxxxxxxx
    Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 8ef499ab3c1ed..955e31860095e 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -273,6 +273,8 @@ static inline void printk_trigger_flush(void)
 }
 #endif
 
+bool this_cpu_in_panic(void);
+
 #ifdef CONFIG_SMP
 extern int __printk_cpu_sync_try_get(void);
 extern void __printk_cpu_sync_wait(void);
diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index ac2d9750e5f81..6c2afee5ef620 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -130,7 +130,6 @@ struct printk_message {
 };
 
 bool other_cpu_in_panic(void);
-bool this_cpu_in_panic(void);
 bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
 			     bool is_extended, bool may_supress);
 
diff --git a/lib/dump_stack.c b/lib/dump_stack.c
index 83471e81501a7..222c6d6c8281a 100644
--- a/lib/dump_stack.c
+++ b/lib/dump_stack.c
@@ -96,15 +96,25 @@ static void __dump_stack(const char *log_lvl)
  */
 asmlinkage __visible void dump_stack_lvl(const char *log_lvl)
 {
+	bool in_panic = this_cpu_in_panic();
 	unsigned long flags;
 
 	/*
 	 * Permit this cpu to perform nested stack dumps while serialising
-	 * against other CPUs
+	 * against other CPUs, unless this CPU is in panic.
+	 *
+	 * When in panic, non-panic CPUs are not permitted to store new
+	 * printk messages so there is no need to synchronize the output.
+	 * This avoids potential deadlock in panic() if another CPU is
+	 * holding and unable to release the printk_cpu_sync.
 	 */
-	printk_cpu_sync_get_irqsave(flags);
+	if (!in_panic)
+		printk_cpu_sync_get_irqsave(flags);
+
 	__dump_stack(log_lvl);
-	printk_cpu_sync_put_irqrestore(flags);
+
+	if (!in_panic)
+		printk_cpu_sync_put_irqrestore(flags);
 }
 EXPORT_SYMBOL(dump_stack_lvl);