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Commit 6c06f072 authored by Nathaniel W. Turner's avatar Nathaniel W. Turner Committed by Alex Elder
Browse files

xfs: copy li_lsn before dropping AIL lock 



Access to log items on the AIL is generally protected by m_ail_lock;
this is particularly needed when we're getting or setting the 64-bit
li_lsn on a 32-bit platform.  This patch fixes a couple places where we
were accessing the log item after dropping the AIL lock on 32-bit
machines.

This can result in a partially-zeroed log->l_tail_lsn if
xfs_trans_ail_delete is racing with xfs_trans_ail_update, and in at
least some cases, this can leave the l_tail_lsn with a zero cycle
number, which means xlog_space_left will think the log is full (unless
CONFIG_XFS_DEBUG is set, in which case we'll trip an ASSERT), leading to
processes stuck forever in xlog_grant_log_space.

Thanks to Adrian VanderSpek for first spotting the race potential and to
Dave Chinner for debug assistance.

Signed-off-by: default avatarNathaniel W. Turner <nate@houseofnate.net>
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
Signed-off-by: default avatarAlex Elder <aelder@sgi.com>
parent 8ec6dba2

fs/xfs/xfs_trans_ail.c

+20 −3
Original line number Diff line number Diff line
@@ -467,6 +467,7 @@ xfs_trans_ail_update( 
{

	xfs_log_item_t		*dlip = NULL;

	xfs_log_item_t		*mlip;	/* ptr to minimum lip */

	xfs_lsn_t		tail_lsn;



	mlip = xfs_ail_min(ailp);
@@ -483,8 +484,16 @@ xfs_trans_ail_update( 


	if (mlip == dlip) {

		mlip = xfs_ail_min(ailp);

		/*

		 * It is not safe to access mlip after the AIL lock is

		 * dropped, so we must get a copy of li_lsn before we do

		 * so.  This is especially important on 32-bit platforms

		 * where accessing and updating 64-bit values like li_lsn

		 * is not atomic.

		 */

		tail_lsn = mlip->li_lsn;

		spin_unlock(&ailp->xa_lock);

		xfs_log_move_tail(ailp->xa_mount, mlip->li_lsn);

		xfs_log_move_tail(ailp->xa_mount, tail_lsn);

	} else {

		spin_unlock(&ailp->xa_lock);

	}
@@ -514,6 +523,7 @@ xfs_trans_ail_delete( 
{

	xfs_log_item_t		*dlip;

	xfs_log_item_t		*mlip;

	xfs_lsn_t		tail_lsn;



	if (lip->li_flags & XFS_LI_IN_AIL) {

		mlip = xfs_ail_min(ailp);
@@ -527,9 +537,16 @@ xfs_trans_ail_delete( 


		if (mlip == dlip) {

			mlip = xfs_ail_min(ailp);

			/*

			 * It is not safe to access mlip after the AIL lock

			 * is dropped, so we must get a copy of li_lsn

			 * before we do so.  This is especially important

			 * on 32-bit platforms where accessing and updating

			 * 64-bit values like li_lsn is not atomic.

			 */

			tail_lsn = mlip ? mlip->li_lsn : 0;

			spin_unlock(&ailp->xa_lock);

			xfs_log_move_tail(ailp->xa_mount,

						(mlip ? mlip->li_lsn : 0));

			xfs_log_move_tail(ailp->xa_mount, tail_lsn);

		} else {

			spin_unlock(&ailp->xa_lock);

		}