GFS2: Support for I/O barriers

This patch adds barrier support to GFS2. There is not a lot of change
really... we just add the barrier flag when we write journal header
blocks. If the underlying device refuses to support them, we fall back
to the previous way of doing things (wait for the I/O and hope) since
there is nothing else we can do. There is no user configuration,
barriers will always be on unless the device refuses to support them.
This seems a reasonable solution to me since this is a correctness
issue.
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/incore.h

@@ -432,6 +432,7 @@ enum {
 	SDF_JOURNAL_CHECKED	= 0,
 	SDF_JOURNAL_LIVE	= 1,
 	SDF_SHUTDOWN		= 2,
+	SDF_NOBARRIERS		= 3,
 };
 
 #define GFS2_FSNAME_LEN		256

fs/gfs2/log.c

@@ -18,6 +18,7 @@
 #include <linux/delay.h>
 #include <linux/kthread.h>
 #include <linux/freezer.h>
+#include <linux/bio.h>
 
 #include "gfs2.h"
 #include "incore.h"
@@ -584,7 +585,6 @@ static void log_write_header(struct gfs2_sbd *sdp, u32 flags, int pull)
 	memset(bh->b_data, 0, bh->b_size);
 	set_buffer_uptodate(bh);
 	clear_buffer_dirty(bh);
-	unlock_buffer(bh);
 
 	gfs2_ail1_empty(sdp, 0);
 	tail = current_tail(sdp);
@@ -601,8 +601,23 @@ static void log_write_header(struct gfs2_sbd *sdp, u32 flags, int pull)
 	hash = gfs2_disk_hash(bh->b_data, sizeof(struct gfs2_log_header));
 	lh->lh_hash = cpu_to_be32(hash);
 
-	set_buffer_dirty(bh);
-	if (sync_dirty_buffer(bh))
+	bh->b_end_io = end_buffer_write_sync;
+	if (test_bit(SDF_NOBARRIERS, &sdp->sd_flags))
+		goto skip_barrier;
+	get_bh(bh);
+	submit_bh(WRITE_BARRIER | (1 << BIO_RW_META), bh);
+	wait_on_buffer(bh);
+	if (buffer_eopnotsupp(bh)) {
+		clear_buffer_eopnotsupp(bh);
+		set_buffer_uptodate(bh);
+		set_bit(SDF_NOBARRIERS, &sdp->sd_flags);
+		lock_buffer(bh);
+skip_barrier:
+		get_bh(bh);
+		submit_bh(WRITE_SYNC | (1 << BIO_RW_META), bh);
+		wait_on_buffer(bh);
+	}
+	if (!buffer_uptodate(bh))
 		gfs2_io_error_bh(sdp, bh);
 	brelse(bh);