linux-kernel - [PATCH printk 2/5] printk: kmsg_dump_rewind

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Date:   Tue, 22 Sep 2020 17:44:13 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: [PATCH printk 2/5] printk: kmsg_dump_rewind_nolock: start from first record

kmsg_dump_rewind_nolock() accesses @clear_seq without any locking.
However, accessing this global variable requires holding
@logbuf_lock. For the _nolock() variant, start from the first record
in the ringbuffer rather than the @clear_seq record.

Signed-off-by: John Ogness <john.ogness@...utronix.de>
---
 kernel/printk/printk.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 00bc1fce3299..cff13b33e926 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3410,11 +3410,12 @@ EXPORT_SYMBOL_GPL(kmsg_dump_get_buffer);
  * kmsg_dump_get_buffer() can be called again and used multiple
  * times within the same dumper.dump() callback.
  *
- * The function is similar to kmsg_dump_rewind(), but grabs no locks.
+ * The function is similar to kmsg_dump_rewind(), but grabs no locks
+ * and starts from the oldest record rather than from @clear_seq.
  */
 void kmsg_dump_rewind_nolock(struct kmsg_dumper *dumper)
 {
-	dumper->cur_seq = clear_seq;
+	dumper->cur_seq = 0;
 	dumper->next_seq = prb_next_seq(prb);
 }
 
@@ -3431,7 +3432,8 @@ void kmsg_dump_rewind(struct kmsg_dumper *dumper)
 	unsigned long flags;
 
 	logbuf_lock_irqsave(flags);
-	kmsg_dump_rewind_nolock(dumper);
+	dumper->cur_seq = clear_seq;
+	dumper->next_seq = prb_next_seq(prb);
 	logbuf_unlock_irqrestore(flags);
 }
 EXPORT_SYMBOL_GPL(kmsg_dump_rewind);
-- 
2.20.1