[PATCH 3.10 08/17] nilfs2: fix deadlock of segment constructor over I_SYNC flag

Greg Kroah-Hartman
2015-02-09T08:35:29Z

From: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

commit 7ef3ff2fea8bf5e4a21cef47ad87710a3d0fdb52 upstream.

Nilfs2 eventually hangs in a stress test with fsstress program. This
issue was caused by the following deadlock over I_SYNC flag between
nilfs_segctor_thread() and writeback_sb_inodes():

nilfs_segctor_thread()
nilfs_segctor_thread_construct()
nilfs_segctor_unlock()
nilfs_dispose_list()
iput()
iput_final()
evict()
inode_wait_for_writeback() * wait for I_SYNC flag

writeback_sb_inodes()
* set I_SYNC flag on inode->i_state
__writeback_single_inode()
do_writepages()
nilfs_writepages()
nilfs_construct_dsync_segment()
nilfs_segctor_sync()
* wait for completion of segment constructor
inode_sync_complete()
* clear I_SYNC flag after __writeback_single_inode() completed

writeback_sb_inodes() calls do_writepages() for dirty inodes after
setting I_SYNC flag on inode->i_state. do_writepages() in turn calls
nilfs_writepages(), which can run segment constructor and wait for its
completion. On the other hand, segment constructor calls iput(), which
can call evict() and wait for the I_SYNC flag on
inode_wait_for_writeback().

Since segment constructor doesn't know when I_SYNC will be set, it
cannot know whether iput() will block or not unless inode->i_nlink has a
non-zero count. We can prevent evict() from being called in iput() by
implementing sop->drop_inode(), but it's not preferable to leave inodes
with i_nlink == 0 for long periods because it even defers file
truncation and inode deallocation. So, this instead resolves the
deadlock by calling iput() asynchronously with a workqueue for inodes
with i_nlink == 0.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Tested-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/nilfs2/nilfs.h | 2 --
fs/nilfs2/segment.c | 44 +++++++++++++++++++++++++++++++++++++++-----
fs/nilfs2/segment.h | 5 +++++
3 files changed, 44 insertions(+), 7 deletions(-)

--- a/fs/nilfs2/nilfs.h
+++ b/fs/nilfs2/nilfs.h
@@ -141,7 +141,6 @@ enum {
* @ti_save: Backup of journal_info field of task_struct
* @ti_flags: Flags
* @ti_count: Nest level
- * @ti_garbage: List of inode to be put when releasing semaphore
*/
struct nilfs_transaction_info {
u32 ti_magic;
@@ -150,7 +149,6 @@ struct nilfs_transaction_info {
one of other filesystems has a bug. */
unsigned short ti_flags;
unsigned short ti_count;
- struct list_head ti_garbage;
};

/* ti_magic */
--- a/fs/nilfs2/segment.c
+++ b/fs/nilfs2/segment.c
@@ -305,7 +305,6 @@ static void nilfs_transaction_lock(struc
ti->ti_count = 0;
ti->ti_save = cur_ti;
ti->ti_magic = NILFS_TI_MAGIC;
- INIT_LIST_HEAD(&ti->ti_garbage);
current->journal_info = ti;

for (;;) {
@@ -332,8 +331,6 @@ static void nilfs_transaction_unlock(str

up_write(&nilfs->ns_segctor_sem);
current->journal_info = ti->ti_save;
- if (!list_empty(&ti->ti_garbage))
- nilfs_dispose_list(nilfs, &ti->ti_garbage, 0);
}

static void *nilfs_segctor_map_segsum_entry(struct nilfs_sc_info *sci,
@@ -746,6 +743,15 @@ static void nilfs_dispose_list(struct th
}
}

+static void nilfs_iput_work_func(struct work_struct *work)
+{
+ struct nilfs_sc_info *sci = container_of(work, struct nilfs_sc_info,
+ sc_iput_work);
+ struct the_nilfs *nilfs = sci->sc_super->s_fs_info;
+
+ nilfs_dispose_list(nilfs, &sci->sc_iput_queue, 0);
+}
+
static int nilfs_test_metadata_dirty(struct the_nilfs *nilfs,
struct nilfs_root *root)
{
@@ -1899,8 +1905,8 @@ static int nilfs_segctor_collect_dirty_f
static void nilfs_segctor_drop_written_files(struct nilfs_sc_info *sci,
struct the_nilfs *nilfs)
{
- struct nilfs_transaction_info *ti = current->journal_info;
struct nilfs_inode_info *ii, *n;
+ int defer_iput = false;

spin_lock(&nilfs->ns_inode_lock);
list_for_each_entry_safe(ii, n, &sci->sc_dirty_files, i_dirty) {
@@ -1911,9 +1917,24 @@ static void nilfs_segctor_drop_written_f
clear_bit(NILFS_I_BUSY, &ii->i_state);
brelse(ii->i_bh);
ii->i_bh = NULL;
- list_move_tail(&ii->i_dirty, &ti->ti_garbage);
+ list_del_init(&ii->i_dirty);
+ if (!ii->vfs_inode.i_nlink) {
+ /*
+ * Defer calling iput() to avoid a deadlock
+ * over I_SYNC flag for inodes with i_nlink == 0
+ */
+ list_add_tail(&ii->i_dirty, &sci->sc_iput_queue);
+ defer_iput = true;
+ } else {
+ spin_unlock(&nilfs->ns_inode_lock);
+ iput(&ii->vfs_inode);
+ spin_lock(&nilfs->ns_inode_lock);
+ }
}
spin_unlock(&nilfs->ns_inode_lock);
+
+ if (defer_iput)
+ schedule_work(&sci->sc_iput_work);
}

/*
@@ -2580,6 +2601,8 @@ static struct nilfs_sc_info *nilfs_segct
INIT_LIST_HEAD(&sci->sc_segbufs);
INIT_LIST_HEAD(&sci->sc_write_logs);
INIT_LIST_HEAD(&sci->sc_gc_inodes);
+ INIT_LIST_HEAD(&sci->sc_iput_queue);
+ INIT_WORK(&sci->sc_iput_work, nilfs_iput_work_func);
init_timer(&sci->sc_timer);

sci->sc_interval = HZ * NILFS_SC_DEFAULT_TIMEOUT;
@@ -2606,6 +2629,8 @@ static void nilfs_segctor_write_out(stru
ret = nilfs_segctor_construct(sci, SC_LSEG_SR);
nilfs_transaction_unlock(sci->sc_super);

+ flush_work(&sci->sc_iput_work);
+
} while (ret && retrycount-- > 0);
}

@@ -2630,6 +2655,9 @@ static void nilfs_segctor_destroy(struct
|| sci->sc_seq_request != sci->sc_seq_done);
spin_unlock(&sci->sc_state_lock);

+ if (flush_work(&sci->sc_iput_work))
+ flag = true;
+
if (flag || !nilfs_segctor_confirm(sci))
nilfs_segctor_write_out(sci);

@@ -2639,6 +2667,12 @@ static void nilfs_segctor_destroy(struct
nilfs_dispose_list(nilfs, &sci->sc_dirty_files, 1);
}

+ if (!list_empty(&sci->sc_iput_queue)) {
+ nilfs_warning(sci->sc_super, __func__,
+ "iput queue is not empty\n");
+ nilfs_dispose_list(nilfs, &sci->sc_iput_queue, 1);
+ }
+
WARN_ON(!list_empty(&sci->sc_segbufs));
WARN_ON(!list_empty(&sci->sc_write_logs));

--- a/fs/nilfs2/segment.h
+++ b/fs/nilfs2/segment.h
@@ -26,6 +26,7 @@
#include <linux/types.h>
#include <linux/fs.h>
#include <linux/buffer_head.h>
+#include <linux/workqueue.h>
#include <linux/nilfs2_fs.h>
#include "nilfs.h"

@@ -92,6 +93,8 @@ struct nilfs_segsum_pointer {
* @sc_nblk_inc: Block count of current generation
* @sc_dirty_files: List of files to be written
* @sc_gc_inodes: List of GC inodes having blocks to be written
+ * @sc_iput_queue: list of inodes for which iput should be done
+ * @sc_iput_work: work struct to defer iput call
* @sc_freesegs: array of segment numbers to be freed
* @sc_nfreesegs: number of segments on @sc_freesegs
* @sc_dsync_inode: inode whose data pages are written for a sync operation
@@ -135,6 +138,8 @@ struct nilfs_sc_info {

struct list_head sc_dirty_files;
struct list_head sc_gc_inodes;
+ struct list_head sc_iput_queue;
+ struct work_struct sc_iput_work;

__u64 *sc_freesegs;
size_t sc_nfreesegs;