[PATCH printk-rework 10/14] um: synchronize kmsg_dumper

[John Ogness]

The kmsg_dumper can be called from any context and CPU, possibly
from multiple CPUs simultaneously. Since a static buffer is used
to retrieve the kernel logs, this buffer must be protected against
simultaneous dumping.

Cc: Richard Weinberger <rich...@nod.at>
Signed-off-by: John Ogness <john.ogn...@linutronix.de>
Reviewed-by: Petr Mladek <pmla...@suse.com>
---
 arch/um/kernel/kmsg_dump.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/um/kernel/kmsg_dump.c b/arch/um/kernel/kmsg_dump.c
index f38349ad00ea..173999422ed8 100644
--- a/arch/um/kernel/kmsg_dump.c
+++ b/arch/um/kernel/kmsg_dump.c
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/kmsg_dump.h>
+#include <linux/spinlock.h>
 #include <linux/console.h>
 #include <shared/init.h>
 #include <shared/kern.h>
@@ -9,8 +10,10 @@ static void kmsg_dumper_stdout(struct kmsg_dumper *dumper,
                                enum kmsg_dump_reason reason,
                                struct kmsg_dumper_iter *iter)
 {
+       static DEFINE_SPINLOCK(lock);
        static char line[1024];
        struct console *con;
+       unsigned long flags;
        size_t len = 0;
 
        /* only dump kmsg when no console is available */
@@ -25,11 +28,16 @@ static void kmsg_dumper_stdout(struct kmsg_dumper *dumper,
        if (con)
                return;
 
+       if (!spin_trylock_irqsave(&lock, flags))
+               return;
+
        printf("kmsg_dump:\n");
        while (kmsg_dump_get_line(iter, true, line, sizeof(line), &len)) {
                line[len] = '\0';
                printf("%s", line);
        }
+
+       spin_unlock_irqrestore(&lock, flags);
 }
 
 static struct kmsg_dumper kmsg_dumper = {
-- 
2.20.1