btrfs: dio iomap DSYNC workaround
iomap dio will run generic_write_sync() for us if the iocb is DSYNC.
This is problematic for us because of 2 reasons:
1. we hold the inode_lock() during this operation, and we take it in
generic_write_sync()
2. we hold a read lock on the dio_sem but take the write lock in fsync
Since we don't want to rip out this code right now, but reworking the
locking is a bit much to do at this point, work around this problem with
this masterpiece of a patch.
First, we clear DSYNC on the iocb so that the iomap stuff doesn't know
that it needs to handle the sync. We save this fact in
current->journal_info, because we need to see do special things once
we're in iomap_begin, and we have no way to pass private information
into iomap_dio_rw().
Next we specify a separate iomap_dio_ops for sync, which implements an
->end_io() callback that gets called when the dio completes. This is
important for AIO, because we really do need to run generic_write_sync()
if we complete asynchronously. However if we're still in the submitting
context when we enter ->end_io() we clear the flag so that the submitter
knows they're the ones that needs to run generic_write_sync().
This is meant to be temporary. We need to work out how to eliminate the
inode_lock() and the dio_sem in our fsync and use another mechanism to
protect these operations.
Tested-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index b626793..4bf1584 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -2023,7 +2023,40 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
atomic_inc(&BTRFS_I(inode)->sync_writers);
if (iocb->ki_flags & IOCB_DIRECT) {
+ /*
+ * 1. We must always clear IOCB_DSYNC in order to not deadlock
+ * in iomap, as it calls generic_write_sync() in this case.
+ * 2. If we are async, we can call iomap_dio_complete() either
+ * in
+ *
+ * 2.1. A worker thread from the last bio completed. In this
+ * case we need to mark the btrfs_dio_data that it is
+ * async in order to call generic_write_sync() properly.
+ * This is handled by setting BTRFS_DIO_SYNC_STUB in the
+ * current->journal_info.
+ * 2.2 The submitter context, because all IO completed
+ * before we exited iomap_dio_rw(). In this case we can
+ * just re-set the IOCB_DSYNC on the iocb and we'll do
+ * the sync below. If our ->end_io() gets called and
+ * current->journal_info is set, then we know we're in
+ * our current context and we will clear
+ * current->journal_info to indicate that we need to
+ * sync below.
+ */
+ if (sync) {
+ ASSERT(current->journal_info == NULL);
+ iocb->ki_flags &= ~IOCB_DSYNC;
+ current->journal_info = BTRFS_DIO_SYNC_STUB;
+ }
num_written = __btrfs_direct_write(iocb, from);
+
+ /*
+ * As stated above, we cleared journal_info, so we need to do
+ * the sync ourselves.
+ */
+ if (sync && current->journal_info == NULL)
+ iocb->ki_flags |= IOCB_DSYNC;
+ current->journal_info = NULL;
} else {
num_written = btrfs_buffered_write(iocb, from);
if (num_written > 0)