2018-05-18 17:56 GMT+02:00 Christoph Hellwig <hch@xxxxxx>:
> On Wed, May 16, 2018 at 10:27:18PM +0200, Andreas Gruenbacher wrote:
>> I/O completion triggers when dio->ref reaches zero, so
>> iomap_dio_bio_end_io will never evaluate submit.waiter before the
>> atomic_dec_and_test in iomap_dio_rw. We probably need an
>> smp_mb__before_atomic() before that atomic_dec_and_test to make sure
>> that iomap_dio_bio_end_io sees the right value of submit.waiter
>> though. Any concerns beyond that?
>
> I suspect in the end union usage might still be fine, but it still
> looks rather dangerous to scramble over the union that late. It will
> also do things like overriding the last_queue pointer which we need
> initialized early for block polling.
>
> I suspect something like the variant below will work much better.
> This moves the wait_for_completion flag into struct iomap_dio
> (in an existing packing hole) and uses that in the completion
> handler. It also always initializes the submit fields to prepare
> for this early, and makes all fallbacks synchronous (if we ever
> get a read fallback that would be the right thing to do).
Yes, that should work.
> diff --git a/fs/iomap.c b/fs/iomap.c
> index e4617ae1f8d6..54b28a58b7e3 100644
> --- a/fs/iomap.c
> +++ b/fs/iomap.c
> @@ -1327,6 +1327,7 @@ struct iomap_dio {
> atomic_t ref;
> unsigned flags;
> int error;
> + bool wait_for_completion;
>
> union {
> /* used during submission and for synchronous completion: */
> @@ -1430,7 +1431,7 @@ static void iomap_dio_bio_end_io(struct bio *bio)
> iomap_dio_set_error(dio, blk_status_to_errno(bio->bi_status));
>
> if (atomic_dec_and_test(&dio->ref)) {
> - if (is_sync_kiocb(dio->iocb)) {
> + if (dio->wait_for_completion) {
> struct task_struct *waiter = dio->submit.waiter;
>
> WRITE_ONCE(dio->submit.waiter, NULL);
> @@ -1646,13 +1647,12 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
> dio->end_io = end_io;
> dio->error = 0;
> dio->flags = 0;
> + dio->wait_for_completion = is_sync_kiocb(iocb);
>
> dio->submit.iter = iter;
> - if (is_sync_kiocb(iocb)) {
> - dio->submit.waiter = current;
> - dio->submit.cookie = BLK_QC_T_NONE;
> - dio->submit.last_queue = NULL;
> - }
> + dio->submit.waiter = current;
> + dio->submit.cookie = BLK_QC_T_NONE;
> + dio->submit.last_queue = NULL;
>
> if (iov_iter_rw(iter) == READ) {
> if (pos >= dio->i_size)
> @@ -1702,7 +1702,7 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
> dio_warn_stale_pagecache(iocb->ki_filp);
> ret = 0;
>
> - if (iov_iter_rw(iter) == WRITE && !is_sync_kiocb(iocb) &&
> + if (iov_iter_rw(iter) == WRITE && !dio->wait_for_completion &&
> !inode->i_sb->s_dio_done_wq) {
> ret = sb_init_dio_done_wq(inode->i_sb);
> if (ret < 0)
> @@ -1717,8 +1717,10 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
> iomap_dio_actor);
> if (ret <= 0) {
> /* magic error code to fall back to buffered I/O */
> - if (ret == -ENOTBLK)
> + if (ret == -ENOTBLK) {
> + dio->wait_for_completion = true;
> ret = 0;
> + }
> break;
> }
> pos += ret;
> @@ -1739,7 +1741,7 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
> dio->flags &= ~IOMAP_DIO_NEED_SYNC;
We still want to make sure the right value of dio->wait_for_completion
is seen by iomap_dio_bio_end_io:
smp_mb__before_atomic();
> if (!atomic_dec_and_test(&dio->ref)) {
> - if (!is_sync_kiocb(iocb))
> + if (!dio->wait_for_completion)
> return -EIOCBQUEUED;
>
> for (;;) {
Thanks,
Andreas
