Re: [PATCH 4/4] ext4: serialize truncate with owerwrite DIO workers

On Wed, 05 Sep 2012 20:59:09 +0400, Dmitry Monakhov [off-list ref] wrote:

On Wed, 5 Sep 2012 17:49:20 +0200, Jan Kara [off-list ref] wrote:

quoted

On Tue 04-09-12 21:36:54, Dmitry Monakhov wrote:

quoted

Jan Kara have spotted interesting issue:
There are  potential data corruption issue with  direct IO overwrites
racing with truncate:
 Like:
  dio write                      truncate_task
  ->ext4_ext_direct_IO
   ->overwrite == 1
    ->down_read(&EXT4_I(inode)->i_data_sem);
    ->mutex_unlock(&inode->i_mutex);
                               ->ext4_setattr()
                                ->inode_dio_wait()
                                ->truncate_setsize()
                                ->ext4_truncate()
                                 ->down_write(&EXT4_I(inode)->i_data_sem);
    ->__blockdev_direct_IO
     ->ext4_get_block
     ->submit_io()
    ->up_read(&EXT4_I(inode)->i_data_sem);
                                 # truncate data blocks, allocate them to
                                 # other inode - bad stuff happens because
                                 # dio is still in flight.

In order to serialize with truncate dio worker should grab extra i_dio_count
reference before drop i_mutex.

  Thanks for the patch. You can add:
Reviewed-by: Jan Kara <jack@suse.cz>

I'm Sorry, but unfortunately in two line patch i've done one mistake :( 
because inode_dio_done() should be before i_mutex will be retaken
otherwise following deadlock happen

ext4_setattr                       ext4_direct_io
                                   mutex_unlock
                                   atomic_inc(inode->i_dio_count)
  mutex_lock(i_mutex)
  inode_dio_wait(inode)  ->BLOCK
                        DEADLOCK<- mutex_lock(i_mutex)
                                   inode_dio_done()

Yeah... This is not just my fault :)
Similar deadlock already exist but happen due to end_io_work

truncate:                          kworker:
ext4_setattr                       ext4_end_io_work
                                    

   mutex_lock(i_mutex)
   inode_dio_wait(inode)  ->BLOCK
                         DEADLOCK<- mutex_trylock()
                                    inode_dio_done()
#TEST_CASE
MNT=/mnt_scrach
unlink $MNT/file
fallocate -l $((1024*1024*1024)) $MNT/file
aio-stress -I 100000 -O -s 100m -n -t 1 -c 10 -o 2 -o 3 $MNT/file &
sleep 3
truncate -s 0 $MNT/file
#TEST_CASE_END

So i'll add your review sing to updated version if you don't mind.

quoted

								Honza

quoted

Signed-off-by: Dmitry Monakhov <redacted>
---
 fs/ext4/inode.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 5a75908..9725acb 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c

@@ -3035,6 +3035,7 @@ static ssize_t ext4_ext_direct_IO(int rw, struct kiocb *iocb,
 		overwrite = *((int *)iocb->private);
 
 		if (overwrite) {
+			atomic_inc(&inode->i_dio_count);
 			down_read(&EXT4_I(inode)->i_data_sem);
 			mutex_unlock(&inode->i_mutex);
 		}

@@ -3134,6 +3135,7 @@ static ssize_t ext4_ext_direct_IO(int rw, struct kiocb *iocb,
 		if (overwrite) {
 			up_read(&EXT4_I(inode)->i_data_sem);
 			mutex_lock(&inode->i_mutex);
+			inode_dio_done(inode);
 		}
 
 		return ret;

-- 
1.7.7.6

-- 
Jan Kara [off-list ref]
SUSE Labs, CR
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