jbd,jbd2: fix oops in jbd2_journal_put_journal_head()

Commit ae4647fb (jbd2: reduce journal_head size) introduced a
regression where we occasionally hit panic in
jbd2_journal_put_journal_head() because of wrong b_jcount. The bug is
caused by gcc making 64-bit access to 32-bit bitfield and thus
clobbering b_jcount.

At least for now, those 8 bytes saved in struct journal_head are not
worth the trouble with gcc bitfield handling so revert that part of
the patch.

Reported-by: EUNBONG SONG <eunb.song@samsung.com>
Reported-by: Tony Luck <tony.luck@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 6 insertions, 2 deletions

diff --git a/include/linux/journal-head.h b/include/linux/journal-head.h
index 13a3da25ff07..98cd41bb39c8 100644
--- a/include/linux/journal-head.h
+++ b/include/linux/journal-head.h
@@ -30,15 +30,19 @@ struct journal_head {
 
         /*
          * Journalling list for this buffer [jbd_lock_bh_state()]
+         * NOTE: We *cannot* combine this with b_modified into a bitfield
+         * as gcc would then (which the C standard allows but which is
+         * very unuseful) make 64-bit accesses to the bitfield and clobber
+         * b_jcount if its update races with bitfield modification.
          */
-        unsigned b_jlist:4;
+        unsigned b_jlist;
 
         /*
          * This flag signals the buffer has been modified by
          * the currently running transaction
          * [jbd_lock_bh_state()]
          */
-        unsigned b_modified:1;
+        unsigned b_modified;
 
         /*
          * Copy of the buffer data frozen for writing to the log.