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Commit e555190d authored by NeilBrown's avatar NeilBrown
Browse files

md/raid1: delay reads that could overtake behind-writes. 



When a raid1 array is configured to support write-behind
on some devices, it normally only reads from other devices.
If all devices are write-behind (because the rest have failed)
it is possible for a read request to be serviced before a
behind-write request, which would appear as data corruption.

So when forced to read from a WriteMostly device, wait for any
write-behind to complete, and don't start any more behind-writes.

Signed-off-by: default avatarNeilBrown <neilb@suse.de>
parent d754c5ae

drivers/md/bitmap.c

+3 −1
Original line number Diff line number Diff line
@@ -1356,7 +1356,8 @@ void bitmap_endwrite(struct bitmap *bitmap, sector_t offset, unsigned long secto 
{

	if (!bitmap) return;

	if (behind) {

		atomic_dec(&bitmap->behind_writes);

		if (atomic_dec_and_test(&bitmap->behind_writes))

			wake_up(&bitmap->behind_wait);

		PRINTK(KERN_DEBUG "dec write-behind count %d/%d\n",

		  atomic_read(&bitmap->behind_writes), bitmap->max_write_behind);

	}
@@ -1680,6 +1681,7 @@ int bitmap_create(mddev_t *mddev) 
	atomic_set(&bitmap->pending_writes, 0);

	init_waitqueue_head(&bitmap->write_wait);

	init_waitqueue_head(&bitmap->overflow_wait);

	init_waitqueue_head(&bitmap->behind_wait);



	bitmap->mddev = mddev;

drivers/md/bitmap.h

+1 −0
Original line number Diff line number Diff line
@@ -240,6 +240,7 @@ struct bitmap { 
	atomic_t pending_writes; /* pending writes to the bitmap file */

	wait_queue_head_t write_wait;

	wait_queue_head_t overflow_wait;

	wait_queue_head_t behind_wait;



	struct sysfs_dirent *sysfs_can_clear;

};

drivers/md/raid1.c

+18 −7
Original line number Diff line number Diff line
@@ -857,6 +857,15 @@ static int make_request(mddev_t *mddev, struct bio * bio) 
		}

		mirror = conf->mirrors + rdisk;



		if (test_bit(WriteMostly, &mirror->rdev->flags) &&

		    bitmap) {

			/* Reading from a write-mostly device must

			 * take care not to over-take any writes

			 * that are 'behind'

			 */

			wait_event(bitmap->behind_wait,

				   atomic_read(&bitmap->behind_writes) == 0);

		}

		r1_bio->read_disk = rdisk;



		read_bio = bio_clone(bio, GFP_NOIO);
@@ -934,10 +943,14 @@ static int make_request(mddev_t *mddev, struct bio * bio) 
		set_bit(R1BIO_Degraded, &r1_bio->state);

	}



	/* do behind I/O ? */

	/* do behind I/O ?

	 * Not if there are too many, or cannot allocate memory,

	 * or a reader on WriteMostly is waiting for behind writes 

	 * to flush */

	if (bitmap &&

	    (atomic_read(&bitmap->behind_writes)

	     < mddev->bitmap_info.max_write_behind) &&

	    !waitqueue_active(&bitmap->behind_wait) &&

	    (behind_pages = alloc_behind_pages(bio)) != NULL)

		set_bit(R1BIO_BehindIO, &r1_bio->state);
@@ -2144,15 +2157,13 @@ static int stop(mddev_t *mddev) 
{

	conf_t *conf = mddev->private;

	struct bitmap *bitmap = mddev->bitmap;

	int behind_wait = 0;



	/* wait for behind writes to complete */

	while (bitmap && atomic_read(&bitmap->behind_writes) > 0) {

		behind_wait++;

		printk(KERN_INFO "raid1: behind writes in progress on device %s, waiting to stop (%d)\n", mdname(mddev), behind_wait);

		set_current_state(TASK_UNINTERRUPTIBLE);

		schedule_timeout(HZ); /* wait a second */

	if (bitmap && atomic_read(&bitmap->behind_writes) > 0) {

		printk(KERN_INFO "raid1: behind writes in progress on device %s, waiting to stop.\n", mdname(mddev));

		/* need to kick something here to make sure I/O goes? */

		wait_event(bitmap->behind_wait,

			   atomic_read(&bitmap->behind_writes) == 0);

	}



	raise_barrier(conf);