lib/dump_stack: move cpu lock to printk.c

dump_stack() implements its own cpu-reentrant spinning lock to
best-effort serialize stack traces in the printk log. However,
there are other functions (such as show_regs()) that can also
benefit from this serialization.

Move the cpu-reentrant spinning lock (cpu lock) into new helper
functions printk_cpu_lock_irqsave()/printk_cpu_unlock_irqrestore()
so that it is available for others as well. For !CONFIG_SMP the
cpu lock is a NOP.

Note that having multiple cpu locks in the system can easily
lead to deadlock. Code needing a cpu lock should use the
printk cpu lock, since the printk cpu lock could be acquired
from any code and any context.

Also note that it is not necessary for a cpu lock to disable
interrupts. However, in upcoming work this cpu lock will be used
for emergency tasks (for example, atomic consoles during kernel
crashes) and any interruptions while holding the cpu lock should
be avoided if possible.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
[pmladek@suse.com: Backported on top of 5.13-rc1.]
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210617095051.4808-2-john.ogness@linutronix.de

1 files changed, 2 insertions, 36 deletions

diff --git a/lib/dump_stack.c b/lib/dump_stack.c
index f5a33b6f773f..5ebf4375fa8c 100644
--- a/lib/dump_stack.c
+++ b/lib/dump_stack.c
@@ -84,50 +84,16 @@ static void __dump_stack(void)
  *
  * Architectures can override this implementation by implementing its own.
  */
-#ifdef CONFIG_SMP
-static atomic_t dump_lock = ATOMIC_INIT(-1);
-
 asmlinkage __visible void dump_stack(void)
 {
 	unsigned long flags;
-	int was_locked;
-	int old;
-	int cpu;
 
 	/*
 	 * Permit this cpu to perform nested stack dumps while serialising
 	 * against other CPUs
 	 */
-retry:
-	local_irq_save(flags);
-	cpu = smp_processor_id();
-	old = atomic_cmpxchg(&dump_lock, -1, cpu);
-	if (old == -1) {
-		was_locked = 0;
-	} else if (old == cpu) {
-		was_locked = 1;
-	} else {
-		local_irq_restore(flags);
-		/*
-		 * Wait for the lock to release before jumping to
-		 * atomic_cmpxchg() in order to mitigate the thundering herd
-		 * problem.
-		 */
-		do { cpu_relax(); } while (atomic_read(&dump_lock) != -1);
-		goto retry;
-	}
-
-	__dump_stack();
-
-	if (!was_locked)
-		atomic_set(&dump_lock, -1);
-
-	local_irq_restore(flags);
-}
-#else
-asmlinkage __visible void dump_stack(void)
-{
+	printk_cpu_lock_irqsave(flags);
 	__dump_stack();
+	printk_cpu_unlock_irqrestore(flags);
 }
-#endif
 EXPORT_SYMBOL(dump_stack);