linux-kernel - [PATCH next v2 2/3] printk: change @clear_seq to atomic64

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Date:   Tue,  1 Dec 2020 21:59:40 +0106
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 next v2 2/3] printk: change @clear_seq to atomic64_t

Currently @clear_seq access is protected by @logbuf_lock. Once
@logbuf_lock is removed some other form of synchronization will be
required. Change the type of @clear_seq to atomic64_t to provide the
synchronization.

Signed-off-by: John Ogness <john.ogness@...utronix.de>
---
 kernel/printk/printk.c | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index fc5e3a7d6d89..e9018c4e1b66 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -403,7 +403,7 @@ static u64 exclusive_console_stop_seq;
 static unsigned long console_dropped;
 
 /* the next printk record to read after the last 'clear' command */
-static u64 clear_seq;
+static atomic64_t clear_seq = ATOMIC64_INIT(0);
 
 #ifdef CONFIG_PRINTK_CALLER
 #define PREFIX_MAX		48
@@ -797,7 +797,7 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
 		 * like issued by 'dmesg -c'. Reading /dev/kmsg itself
 		 * changes no global state, and does not clear anything.
 		 */
-		user->seq = clear_seq;
+		user->seq = atomic64_read(&clear_seq);
 		break;
 	case SEEK_END:
 		/* after the last record */
@@ -914,6 +914,9 @@ void log_buf_vmcoreinfo_setup(void)
 	 * parse it and detect any changes to structure down the line.
 	 */
 
+	VMCOREINFO_SIZE(atomic64_t);
+	VMCOREINFO_TYPE_OFFSET(atomic64_t, counter);
+
 	VMCOREINFO_STRUCT_SIZE(printk_ringbuffer);
 	VMCOREINFO_OFFSET(printk_ringbuffer, desc_ring);
 	VMCOREINFO_OFFSET(printk_ringbuffer, text_data_ring);
@@ -1476,6 +1479,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
 	struct printk_info info;
 	unsigned int line_count;
 	struct printk_record r;
+	u64 clr_seq;
 	char *text;
 	int len = 0;
 	u64 seq;
@@ -1487,15 +1491,17 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
 
 	time = printk_time;
 	logbuf_lock_irq();
+	clr_seq = atomic64_read(&clear_seq);
+
 	/*
 	 * Find first record that fits, including all following records,
 	 * into the user-provided buffer for this dump.
 	 */
-	prb_for_each_info(clear_seq, prb, seq, &info, &line_count)
+	prb_for_each_info(clr_seq, prb, seq, &info, &line_count)
 		len += get_record_print_text_size(&info, line_count, true, time);
 
 	/* move first record forward until length fits into the buffer */
-	prb_for_each_info(clear_seq, prb, seq, &info, &line_count) {
+	prb_for_each_info(clr_seq, prb, seq, &info, &line_count) {
 		if (len <= size)
 			break;
 		len -= get_record_print_text_size(&info, line_count, true, time);
@@ -1526,7 +1532,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
 	}
 
 	if (clear)
-		clear_seq = seq;
+		atomic64_set(&clear_seq, seq);
 	logbuf_unlock_irq();
 
 	kfree(text);
@@ -1536,7 +1542,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
 static void syslog_clear(void)
 {
 	logbuf_lock_irq();
-	clear_seq = prb_next_seq(prb);
+	atomic64_set(&clear_seq, prb_next_seq(prb));
 	logbuf_unlock_irq();
 }
 
@@ -3205,7 +3211,7 @@ void kmsg_dump(enum kmsg_dump_reason reason)
 		dumper->active = true;
 
 		logbuf_lock_irqsave(flags);
-		dumper->cur_seq = clear_seq;
+		dumper->cur_seq = atomic64_read(&clear_seq);
 		dumper->next_seq = prb_next_seq(prb);
 		logbuf_unlock_irqrestore(flags);
 
@@ -3412,7 +3418,7 @@ EXPORT_SYMBOL_GPL(kmsg_dump_get_buffer);
  */
 void kmsg_dump_rewind_nolock(struct kmsg_dumper *dumper)
 {
-	dumper->cur_seq = clear_seq;
+	dumper->cur_seq = atomic64_read(&clear_seq);
 	dumper->next_seq = prb_next_seq(prb);
 }
 
-- 
2.20.1