From 491caa43639abcffaa645fbab372a7ef4ce2975c Mon Sep 17 00:00:00 2001 From: Jeff Moyer Date: Mon, 5 Mar 2012 10:29:52 -0500 Subject: [PATCH] ext4: fix race between sync and completed io work The following command line will leave the aio-stress process unkillable on an ext4 file system (in my case, mounted on /mnt/test): aio-stress -t 20 -s 10 -O -S -o 2 -I 1000 /mnt/test/aiostress.3561.4 /mnt/test/aiostress.3561.4.20 /mnt/test/aiostress.3561.4.19 /mnt/test/aiostress.3561.4.18 /mnt/test/aiostress.3561.4.17 /mnt/test/aiostress.3561.4.16 /mnt/test/aiostress.3561.4.15 /mnt/test/aiostress.3561.4.14 /mnt/test/aiostress.3561.4.13 /mnt/test/aiostress.3561.4.12 /mnt/test/aiostress.3561.4.11 /mnt/test/aiostress.3561.4.10 /mnt/test/aiostress.3561.4.9 /mnt/test/aiostress.3561.4.8 /mnt/test/aiostress.3561.4.7 /mnt/test/aiostress.3561.4.6 /mnt/test/aiostress.3561.4.5 /mnt/test/aiostress.3561.4.4 /mnt/test/aiostress.3561.4.3 /mnt/test/aiostress.3561.4.2 This is using the aio-stress program from the xfstests test suite. That particular command line tells aio-stress to do random writes to 20 files from 20 threads (one thread per file). The files are NOT preallocated, so you will get writes to random offsets within the file, thus creating holes and extending i_size. It also opens the file with O_DIRECT and O_SYNC. On to the problem. When an I/O requires unwritten extent conversion, it is queued onto the completed_io_list for the ext4 inode. Two code paths will pull work items from this list. The first is the ext4_end_io_work routine, and the second is ext4_flush_completed_IO, which is called via the fsync path (and O_SYNC handling, as well). There are two issues I've found in these code paths. First, if the fsync path beats the work routine to a particular I/O, the work routine will free the io_end structure! It does not take into account the fact that the io_end may still be in use by the fsync path. I've fixed this issue by adding yet another IO_END flag, indicating that the io_end is being processed by the fsync path. The second problem is that the work routine will make an assignment to io->flag outside of the lock. I have witnessed this result in a hang at umount. Moving the flag setting inside the lock resolved that problem. The problem was introduced by commit b82e384c7b ("ext4: optimize locking for end_io extent conversion"), which first appeared in 3.2. As such, the fix should be backported to that release (probably along with the unwritten extent conversion race fix). Signed-off-by: Jeff Moyer Signed-off-by: "Theodore Ts'o" CC: stable@kernel.org --- fs/ext4/ext4.h | 1 + fs/ext4/fsync.c | 2 ++ fs/ext4/page-io.c | 9 +++++++-- 3 files changed, 10 insertions(+), 2 deletions(-) diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h index 6481e3ca3528..37e7d8b66c99 100644 --- a/fs/ext4/ext4.h +++ b/fs/ext4/ext4.h @@ -185,6 +185,7 @@ struct mpage_da_data { #define EXT4_IO_END_ERROR 0x0002 #define EXT4_IO_END_QUEUED 0x0004 #define EXT4_IO_END_DIRECT 0x0008 +#define EXT4_IO_END_IN_FSYNC 0x0010 struct ext4_io_page { struct page *p_page; diff --git a/fs/ext4/fsync.c b/fs/ext4/fsync.c index 00a2cb753efd..bb6c7d811313 100644 --- a/fs/ext4/fsync.c +++ b/fs/ext4/fsync.c @@ -89,6 +89,7 @@ int ext4_flush_completed_IO(struct inode *inode) io = list_entry(ei->i_completed_io_list.next, ext4_io_end_t, list); list_del_init(&io->list); + io->flag |= EXT4_IO_END_IN_FSYNC; /* * Calling ext4_end_io_nolock() to convert completed * IO to written. @@ -108,6 +109,7 @@ int ext4_flush_completed_IO(struct inode *inode) if (ret < 0) ret2 = ret; spin_lock_irqsave(&ei->i_completed_io_lock, flags); + io->flag &= ~EXT4_IO_END_IN_FSYNC; } spin_unlock_irqrestore(&ei->i_completed_io_lock, flags); return (ret2 < 0) ? ret2 : 0; diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c index 9e1b8eb1e7ac..dcdeef169a69 100644 --- a/fs/ext4/page-io.c +++ b/fs/ext4/page-io.c @@ -129,12 +129,18 @@ static void ext4_end_io_work(struct work_struct *work) unsigned long flags; spin_lock_irqsave(&ei->i_completed_io_lock, flags); + if (io->flag & EXT4_IO_END_IN_FSYNC) + goto requeue; if (list_empty(&io->list)) { spin_unlock_irqrestore(&ei->i_completed_io_lock, flags); goto free; } if (!mutex_trylock(&inode->i_mutex)) { + bool was_queued; +requeue: + was_queued = !!(io->flag & EXT4_IO_END_QUEUED); + io->flag |= EXT4_IO_END_QUEUED; spin_unlock_irqrestore(&ei->i_completed_io_lock, flags); /* * Requeue the work instead of waiting so that the work @@ -147,9 +153,8 @@ static void ext4_end_io_work(struct work_struct *work) * yield the cpu if it sees an end_io request that has already * been requeued. */ - if (io->flag & EXT4_IO_END_QUEUED) + if (was_queued) yield(); - io->flag |= EXT4_IO_END_QUEUED; return; } list_del_init(&io->list);