[PATCH] xfs/log: protect the logging content under xc_ctx_lock

[Pingfan Liu <kernelfans@gmail.com>]

xc_cil_lock is not enough to protect the integrity of a trans logging.
Taking the scenario:
  cpuA                                 cpuB                          cpuC

  xlog_cil_insert_format_items()

  spin_lock(&cil->xc_cil_lock)
  link transA's items to xc_cil,
     including item1
  spin_unlock(&cil->xc_cil_lock)
                                                                      xlog_cil_push() fetches transA's item under xc_cil_lock
                                       issue transB, modify item1
                                                                      xlog_write(), but now, item1 contains content from transB and we have a broken transA

Survive this race issue by putting under the protection of xc_ctx_lock.
Meanwhile the xc_cil_lock can be dropped as xc_ctx_lock does it against
xlog_cil_insert_items()

Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
Cc: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: Brian Foster <bfoster@redhat.com>
To: linux-xfs@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
---
 fs/xfs/xfs_log_cil.c | 35 +++++++++++++++++++----------------
 1 file changed, 19 insertions(+), 16 deletions(-)

diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
index 004af09..f8df3b5 100644
--- a/fs/xfs/xfs_log_cil.c
+++ b/fs/xfs/xfs_log_cil.c
@@ -723,22 +723,6 @@ xlog_cil_push(
 	 */
 	lv = NULL;
 	num_iovecs = 0;
-	spin_lock(&cil->xc_cil_lock);
-	while (!list_empty(&cil->xc_cil)) {
-		struct xfs_log_item	*item;
-
-		item = list_first_entry(&cil->xc_cil,
-					struct xfs_log_item, li_cil);
-		list_del_init(&item->li_cil);
-		if (!ctx->lv_chain)
-			ctx->lv_chain = item->li_lv;
-		else
-			lv->lv_next = item->li_lv;
-		lv = item->li_lv;
-		item->li_lv = NULL;
-		num_iovecs += lv->lv_niovecs;
-	}
-	spin_unlock(&cil->xc_cil_lock);
 
 	/*
 	 * initialise the new context and attach it to the CIL. Then attach
@@ -783,6 +767,25 @@ xlog_cil_push(
 	up_write(&cil->xc_ctx_lock);
 
 	/*
+	 * cil->xc_cil_lock around this loop can be dropped, since xc_ctx_lock
+	 * protects us against xlog_cil_insert_items().
+	 */
+	while (!list_empty(&cil->xc_cil)) {
+		struct xfs_log_item	*item;
+
+		item = list_first_entry(&cil->xc_cil,
+					struct xfs_log_item, li_cil);
+		list_del_init(&item->li_cil);
+		if (!ctx->lv_chain)
+			ctx->lv_chain = item->li_lv;
+		else
+			lv->lv_next = item->li_lv;
+		lv = item->li_lv;
+		item->li_lv = NULL;
+		num_iovecs += lv->lv_niovecs;
+	}
+
+	/*
 	 * Build a checkpoint transaction header and write it to the log to
 	 * begin the transaction. We need to account for the space used by the
 	 * transaction header here as it is not accounted for in xlog_write().
-- 
2.7.5