jbd2: refine waiting for shadow buffers

Currently when we add a buffer to a transaction, we wait until the
buffer is removed from BJ_Shadow list (so that we prevent any changes
to the buffer that is just written to the journal).  This can take
unnecessarily long as a lot happens between the time the buffer is
submitted to the journal and the time when we remove the buffer from
BJ_Shadow list.  (e.g.  We wait for all data buffers in the
transaction, we issue a cache flush, etc.)  Also this creates a
dependency of do_get_write_access() on transaction commit (namely
waiting for data IO to complete) which we want to avoid when
implementing transaction reservation.

So we modify commit code to set new BH_Shadow flag when temporary
shadowing buffer is created and we clear that flag once IO on that
buffer is complete.  This allows do_get_write_access() to wait only
for BH_Shadow bit and thus removes the dependency on data IO
completion.

Reviewed-by: Zheng Liu <wenqing.lz@taobao.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 9 insertions, 9 deletions

diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
index 7c6f7eea2316..d73a0d808ec1 100644
--- a/fs/jbd2/commit.c
+++ b/fs/jbd2/commit.c
@@ -30,15 +30,22 @@
 #include <trace/events/jbd2.h>
 
 /*
- * Default IO end handler for temporary BJ_IO buffer_heads.
+ * IO end handler for temporary buffer_heads handling writes to the journal.
  */
 static void journal_end_buffer_io_sync(struct buffer_head *bh, int uptodate)
 {
+        struct buffer_head *orig_bh = bh->b_private;
+
         BUFFER_TRACE(bh, "");
         if (uptodate)
                 set_buffer_uptodate(bh);
         else
                 clear_buffer_uptodate(bh);
+        if (orig_bh) {
+                clear_bit_unlock(BH_Shadow, &orig_bh->b_state);
+                smp_mb__after_clear_bit();
+                wake_up_bit(&orig_bh->b_state, BH_Shadow);
+        }
         unlock_buffer(bh);
 }
 
@@ -832,6 +839,7 @@ start_journal_io:
                 bh = jh2bh(jh);
                 clear_buffer_jwrite(bh);
                 J_ASSERT_BH(bh, buffer_jbddirty(bh));
+                J_ASSERT_BH(bh, !buffer_shadow(bh));
 
                 /* The metadata is now released for reuse, but we need
                    to remember it against this transaction so that when
@@ -839,14 +847,6 @@ start_journal_io:
                    required. */
                 JBUFFER_TRACE(jh, "file as BJ_Forget");
                 jbd2_journal_file_buffer(jh, commit_transaction, BJ_Forget);
-                /*
-                 * Wake up any transactions which were waiting for this IO to
-                 * complete. The barrier must be here so that changes by
-                 * jbd2_journal_file_buffer() take effect before wake_up_bit()
-                 * does the waitqueue check.
-                 */
-                smp_mb();
-                wake_up_bit(&bh->b_state, BH_Unshadow);
                 JBUFFER_TRACE(jh, "brelse shadowed buffer");
                 __brelse(bh);
         }