Patchwork ext4: fix race between sync and completed io work

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Submitter Jeff Moyer
Date Feb. 29, 2012, 9:29 p.m.
Message ID <x49mx81s4zr.fsf@segfault.boston.devel.redhat.com>
Download mbox | patch
Permalink /patch/143822/
State Accepted
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Comments

Jeff Moyer - Feb. 29, 2012, 9:29 p.m.
Hi,

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

Note that this is the aio-stress program from the xfstests-dev 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 the following commit, which first appeared
in 3.2:

commit b82e384c7bb9a19036b4daf58fa216df7cd48aa0
Author: Theodore Ts'o <tytso@mit.edu>
Date:   Mon Oct 31 10:56:32 2011 -0400

    ext4: optimize locking for end_io extent conversion

As such, the fix should be backported to that release (probably along
with the unwritten extent conversion race fix I posted a little while
back).

I will submit the test case above to xfstests in the near future.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
CC: stable@kernel.org

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Theodore Ts'o - March 5, 2012, 3:30 p.m.
On Wed, Feb 29, 2012 at 04:29:28PM -0500, Jeff Moyer wrote:
> Hi,
> 
> The following command line will leave the aio-stress process unkillable
> on an ext4 file system (in my case, mounted on /mnt/test)....

Thanks, applied.

					- Ted
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Patch

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 2d55d7c..3ce6a0c 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 00a2cb7..bb6c7d8 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 9e1b8eb..dcdeef1 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);