On Thu, Nov 15, 2018 at 12:51:28PM -0700, Jens Axboe wrote: > Ensure that writes to the dio/bio waiter field are ordered > correctly. With the smp_rmb() before the READ_ONCE() check, > we should be able to use a more relaxed ordering for the > task state setting. We don't need a heavier barrier on > the wakeup side after writing the waiter field, since we > either going to be in the task we care about, or go through > wake_up_process() which implies a strong enough barrier. > > For the core poll helper, the task state setting don't need > to imply any atomics, as it's the current task itself that > is being modified and we're not going to sleep. > > Signed-off-by: Jens Axboe <[email protected]> > --- > block/blk-mq.c | 4 ++-- > fs/block_dev.c | 9 +++++++-- > fs/iomap.c | 4 +++- > mm/page_io.c | 4 +++- > 4 files changed, 15 insertions(+), 6 deletions(-) > > diff --git a/block/blk-mq.c b/block/blk-mq.c > index 32b246ed44c0..7fc4abb4cc36 100644 > --- a/block/blk-mq.c > +++ b/block/blk-mq.c > @@ -3331,12 +3331,12 @@ static bool __blk_mq_poll(struct blk_mq_hw_ctx *hctx, > struct request *rq) > ret = q->mq_ops->poll(hctx, rq->tag); > if (ret > 0) { > hctx->poll_success++; > - set_current_state(TASK_RUNNING); > + __set_current_state(TASK_RUNNING); > return true; > } > > if (signal_pending_state(state, current)) > - set_current_state(TASK_RUNNING); > + __set_current_state(TASK_RUNNING); > > if (current->state == TASK_RUNNING) > return true; > diff --git a/fs/block_dev.c b/fs/block_dev.c > index c039abfb2052..5b754f84c814 100644 > --- a/fs/block_dev.c > +++ b/fs/block_dev.c > @@ -237,9 +237,12 @@ __blkdev_direct_IO_simple(struct kiocb *iocb, struct > iov_iter *iter, > > qc = submit_bio(&bio); > for (;;) { > - set_current_state(TASK_UNINTERRUPTIBLE); > + __set_current_state(TASK_UNINTERRUPTIBLE); > + > + smp_rmb(); > if (!READ_ONCE(bio.bi_private)) > break; > + > if (!(iocb->ki_flags & IOCB_HIPRI) || > !blk_poll(bdev_get_queue(bdev), qc)) > io_schedule(); > @@ -403,7 +406,9 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter > *iter, int nr_pages) > return -EIOCBQUEUED; > > for (;;) { > - set_current_state(TASK_UNINTERRUPTIBLE); > + __set_current_state(TASK_UNINTERRUPTIBLE); > + > + smp_rmb(); > if (!READ_ONCE(dio->waiter)) > break; > > diff --git a/fs/iomap.c b/fs/iomap.c > index f61d13dfdf09..3373ea4984d9 100644 > --- a/fs/iomap.c > +++ b/fs/iomap.c > @@ -1888,7 +1888,9 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter, > return -EIOCBQUEUED; > > for (;;) { > - set_current_state(TASK_UNINTERRUPTIBLE); > + __set_current_state(TASK_UNINTERRUPTIBLE); > + > + smp_rmb(); > if (!READ_ONCE(dio->submit.waiter)) > break; > > diff --git a/mm/page_io.c b/mm/page_io.c > index d4d1c89bcddd..008f6d00c47c 100644 > --- a/mm/page_io.c > +++ b/mm/page_io.c > @@ -405,7 +405,9 @@ int swap_readpage(struct page *page, bool synchronous) > bio_get(bio); > qc = submit_bio(bio); > while (synchronous) { > - set_current_state(TASK_UNINTERRUPTIBLE); > + __set_current_state(TASK_UNINTERRUPTIBLE); > + > + smp_rmb(); > if (!READ_ONCE(bio->bi_private))
I think any smp_rmb() should have a big fact comment explaining it. Also to help stupid people like me that dont understand why we even need it here given the READ_ONCE below.
