Commit 7d7a23a9 authored by John Ogness's avatar John Ogness Committed by Petr Mladek
Browse files

printk: use seqcount_latch for clear_seq 



kmsg_dump_rewind_nolock() locklessly reads @clear_seq. However,
this is not done atomically. Since @clear_seq is 64-bit, this
cannot be an atomic operation for all platforms. Therefore, use
a seqcount_latch to allow readers to always read a consistent
value.

Signed-off-by: default avatarJohn Ogness <john.ogness@linutronix.de>
Reviewed-by: default avatarPetr Mladek <pmladek@suse.com>
Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210303101528.29901-9-john.ogness@linutronix.de
parent cf5b0208

kernel/printk/printk.c

+50 −8
Original line number Diff line number Diff line
@@ -402,8 +402,21 @@ static u64 console_seq; 
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;

struct latched_seq {

	seqcount_latch_t	latch;

	u64			val[2];

};



/*

 * The next printk record to read after the last 'clear' command. There are

 * two copies (updated with seqcount_latch) so that reads can locklessly

 * access a valid value. Writers are synchronized by @logbuf_lock.

 */

static struct latched_seq clear_seq = {

	.latch		= SEQCNT_LATCH_ZERO(clear_seq.latch),

	.val[0]		= 0,

	.val[1]		= 0,

};



#ifdef CONFIG_PRINTK_CALLER

#define PREFIX_MAX		48
@@ -457,6 +470,31 @@ bool printk_percpu_data_ready(void) 
	return __printk_percpu_data_ready;

}



/* Must be called under logbuf_lock. */

static void latched_seq_write(struct latched_seq *ls, u64 val)

{

	raw_write_seqcount_latch(&ls->latch);

	ls->val[0] = val;

	raw_write_seqcount_latch(&ls->latch);

	ls->val[1] = val;

}



/* Can be called from any context. */

static u64 latched_seq_read_nolock(struct latched_seq *ls)

{

	unsigned int seq;

	unsigned int idx;

	u64 val;



	do {

		seq = raw_read_seqcount_latch(&ls->latch);

		idx = seq & 0x1;

		val = ls->val[idx];

	} while (read_seqcount_latch_retry(&ls->latch, seq));



	return val;

}



/* Return log buffer address */

char *log_buf_addr_get(void)

{
@@ -801,7 +839,7 @@ 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 = latched_seq_read_nolock(&clear_seq);

		break;

	case SEEK_END:

		/* after the last record */
@@ -960,6 +998,9 @@ void log_buf_vmcoreinfo_setup(void) 


	VMCOREINFO_SIZE(atomic_long_t);

	VMCOREINFO_TYPE_OFFSET(atomic_long_t, counter);



	VMCOREINFO_STRUCT_SIZE(latched_seq);

	VMCOREINFO_OFFSET(latched_seq, val);

}

#endif
@@ -1557,7 +1598,8 @@ static int syslog_print_all(char __user *buf, int size, bool clear) 
	 * Find first record that fits, including all following records,

	 * into the user-provided buffer for this dump.

	 */

	seq = find_first_fitting_seq(clear_seq, -1, size, true, time);

	seq = find_first_fitting_seq(latched_seq_read_nolock(&clear_seq), -1,

				     size, true, time);



	prb_rec_init_rd(&r, &info, text, CONSOLE_LOG_MAX);
@@ -1584,7 +1626,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear) 
	}



	if (clear)

		clear_seq = seq;

		latched_seq_write(&clear_seq, seq);

	logbuf_unlock_irq();



	kfree(text);
@@ -1594,7 +1636,7 @@ 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);

	latched_seq_write(&clear_seq, prb_next_seq(prb));

	logbuf_unlock_irq();

}
@@ -3336,7 +3378,7 @@ void kmsg_dump(enum kmsg_dump_reason reason) 
		dumper->active = true;



		logbuf_lock_irqsave(flags);

		dumper->cur_seq = clear_seq;

		dumper->cur_seq = latched_seq_read_nolock(&clear_seq);

		dumper->next_seq = prb_next_seq(prb);

		logbuf_unlock_irqrestore(flags);
@@ -3534,7 +3576,7 @@ EXPORT_SYMBOL_GPL(kmsg_dump_get_buffer); 
 */

void kmsg_dump_rewind_nolock(struct kmsg_dumper *dumper)

{

	dumper->cur_seq = clear_seq;

	dumper->cur_seq = latched_seq_read_nolock(&clear_seq);

	dumper->next_seq = prb_next_seq(prb);

}