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@xxxxxxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
---
include/linux/journal-head.h | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
Ted, this should fix the regression reported by EUNBONG SONG. Please apply.
diff --git a/include/linux/journal-head.h b/include/linux/journal-head.h
index 13a3da2..3594c74 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 (correctly according to C++11) 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.
--
1.8.1.4
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