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>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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