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Subject[RFC 5/5] dio: only call aio_complete() after returning -EIOCBQUEUED
Date
dio: only call aio_complete() after returning -EIOCBQUEUED

The only time it is safe to call aio_complete() is when the ->ki_retry function
returns -EIOCBQUEUED to the AIO core. direct_io_worker() has historically done
this by relying on its caller to translate positive return codes into
-EIOCBQUEUED for the aio case. It did this by trying to keep conditionals in
sync. direct_io_worker() knew when finished_one_bio() was going to call
aio_complete(). It would reverse the test and wait and free the dio in the
cases it thought that finished_one_bio() wasn't going to.

Not surprisingly, it ended up getting it wrong. 'ret' could be a negative
errno from the submission path but it failed to communicate this to
finished_one_bio(). direct_io_worker() would return < 0, it's callers wouldn't
raise -EIOCBQUEUED, and aio_complete() would be called. In the future
finished_one_bio()'s tests wouldn't reflect this and aio_complete() would be
called for a second time which can manifest as an oops.

The previous cleanups have whittled the sync and async completion paths down to
the point where we can collapse them and clearly reassert the invariant that we
must only call aio_complete() after returning -EIOCBQUEUED. direct_io_worker()
will only return -EIOCBQUEUED when it is not the last to drop the dio refcount
and the aio bio completion path will only call aio_complete() when it is the
last to drop the dio refcount. direct_io_worker() can ensure that it is the
last to drop the reference count by waiting for bios to drain. It does this
for sync ops, of course, and for partial dio writes that must fall back to
buffered and for aio ops that saw errors during submission.

This means that operations that end up waiting, even if they were issued as aio
ops, will not call aio_complete() from dio. Instead we return the return code
of the operation and let the aio core call aio_complete(). This is purposely
done to fix a bug where AIO DIO file extensions would call aio_complete()
before their callers have a chance to update i_size.

Now that direct_io_worker() is explicitly returning -EIOCBQUEUED its callers no
longer have to translate for it.

Signed-off-by: Zach Brown <zach.brown@oracle.com>
---

fs/direct-io.c | 92 ++++++++++++++++++-----------------------------
mm/filemap.c | 4 --
2 files changed, 36 insertions(+), 60 deletions(-)

Index: 2.6.18-rc6-dio-cleanup/fs/direct-io.c
===================================================================
--- 2.6.18-rc6-dio-cleanup.orig/fs/direct-io.c
+++ 2.6.18-rc6-dio-cleanup/fs/direct-io.c
@@ -225,6 +225,15 @@ static int dio_complete(struct dio *dio,
{
ssize_t transferred = 0;

+ /*
+ * AIO submission can race with bio completion to get here while
+ * expecting to have the last io completed by bio completion.
+ * In that case -EIOCBQUEUED is in fact not an error we want
+ * to preserve through this call.
+ */
+ if (ret == -EIOCBQUEUED)
+ ret = 0;
+
if (dio->result) {
transferred = dio->result;

@@ -250,24 +259,6 @@ static int dio_complete(struct dio *dio,
return ret;
}

-/*
- * Called when a BIO has been processed. If the count goes to zero then IO is
- * complete and we can signal this to the AIO layer.
- */
-static void dio_complete_aio(struct dio *dio)
-{
- int ret;
-
- ret = dio_complete(dio, dio->iocb->ki_pos, 0);
-
- /* Complete AIO later if falling back to buffered i/o */
- if (dio->result == dio->size ||
- ((dio->rw == READ) && dio->result)) {
- aio_complete(dio->iocb, ret, 0);
- kfree(dio);
- }
-}
-
static int dio_bio_complete(struct dio *dio, struct bio *bio);
/*
* Asynchronous IO callback.
@@ -289,8 +280,11 @@ static int dio_bio_end_aio(struct bio *b
if (remaining == 1 && waiter_holds_ref)
wake_up_process(dio->waiter);

- if (remaining == 0)
- dio_complete_aio(dio);
+ if (remaining == 0) {
+ int ret = dio_complete(dio, dio->iocb->ki_pos, 0);
+ aio_complete(dio->iocb, ret, 0);
+ kfree(dio);
+ }

return 0;
}
@@ -1074,47 +1068,33 @@ direct_io_worker(int rw, struct kiocb *i
mutex_unlock(&dio->inode->i_mutex);

/*
- * OK, all BIOs are submitted, so we can decrement bio_count to truly
- * reflect the number of to-be-processed BIOs.
- */
- if (dio->is_async) {
- int should_wait = 0;
-
- if (dio->result < dio->size && (rw & WRITE)) {
- dio->waiter = current;
- should_wait = 1;
- }
- if (ret == 0)
- ret = dio->result;
-
- if (should_wait)
- dio_await_completion(dio);
-
- /* this can free the dio */
- if (atomic_dec_and_test(&dio->refcount))
- dio_complete_aio(dio);
+ * The only time we want to leave bios in flight is when a successful
+ * partial aio read or full aio write have been setup. In that case
+ * bio completion will call aio_complete. The only time it's safe to
+ * call aio_complete is when we return -EIOCBQUEUED, so we key on that.
+ * This had *better* be the only place that raises -EIOCBQUEUED.
+ */
+ BUG_ON(ret == -EIOCBQUEUED);
+ if (dio->is_async && ret == 0 && dio->result &&
+ ((rw & READ) || (dio->result == dio->size)))
+ ret = -EIOCBQUEUED;

- if (should_wait)
- kfree(dio);
- } else {
+ if (ret != -EIOCBQUEUED)
dio_await_completion(dio);

+ /*
+ * Sync will always be dropping the final ref and completing the
+ * operation. AIO can if it was a broken operation described above
+ * or in fact if all the bios race to complete before we get here.
+ * In that case dio_complete() translates the EIOCBQUEUED into
+ * the proper return code that the caller will hand to aio_complete().
+ */
+ if (atomic_dec_and_test(&dio->refcount)) {
ret = dio_complete(dio, offset, ret);
+ kfree(dio);
+ } else
+ BUG_ON(ret != -EIOCBQUEUED);

- /* We could have also come here on an AIO file extend */
- if (!is_sync_kiocb(iocb) && (rw & WRITE) &&
- ret >= 0 && dio->result == dio->size)
- /*
- * For AIO writes where we have completed the
- * i/o, we have to mark the the aio complete.
- */
- aio_complete(iocb, ret, 0);
-
- if (atomic_dec_and_test(&dio->refcount))
- kfree(dio);
- else
- BUG();
- }
return ret;
}

Index: 2.6.18-rc6-dio-cleanup/mm/filemap.c
===================================================================
--- 2.6.18-rc6-dio-cleanup.orig/mm/filemap.c
+++ 2.6.18-rc6-dio-cleanup/mm/filemap.c
@@ -1175,8 +1175,6 @@ __generic_file_aio_read(struct kiocb *io
if (pos < size) {
retval = generic_file_direct_IO(READ, iocb,
iov, pos, nr_segs);
- if (retval > 0 && !is_sync_kiocb(iocb))
- retval = -EIOCBQUEUED;
if (retval > 0)
*ppos = pos + retval;
}
@@ -2053,8 +2051,6 @@ generic_file_direct_write(struct kiocb *
if (err < 0)
written = err;
}
- if (written == count && !is_sync_kiocb(iocb))
- written = -EIOCBQUEUED;
return written;
}
EXPORT_SYMBOL(generic_file_direct_write);
-
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