From 6a9a92f4ef05bb3e94bbfe123c21482fa5da9866 Mon Sep 17 00:00:00 2001 From: Lars Ellenberg Date: Thu, 6 Oct 2011 17:10:34 +0200 Subject: [PATCH] drbd: fix harmless race to not trigger an ASSERT We have one pre-allocated page to do certain synchronous meta data IO with, using it is serialized like so: drbd_md_get_buffer(); drbd_md_sync_page_io(); drbd_md_sync_page_io(); ... drbd_md_put_buffer(); In drbd_md_sync_page_io() there is an ASSERT(atomic_read(&mdev->md_io_in_use) == 1); We want to be able to timeout on unresponsive lower level devices, so we can "detach" in that case. Inside drbd_md_sync_page_io() we grab an extra reference, to not have a dangling pointer in case a delayed IO eventually does still complete, even after we "detached" already. We need to put the extra reference before we signal completion from the completion handler, or the second drbd_md_sync_page_io() above may trigger the assert (reference count still 2). Signed-off-by: Philipp Reisner Signed-off-by: Lars Ellenberg --- drivers/block/drbd/drbd_worker.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) diff --git a/drivers/block/drbd/drbd_worker.c b/drivers/block/drbd/drbd_worker.c index 933091ffefca..62bde5ae17f7 100644 --- a/drivers/block/drbd/drbd_worker.c +++ b/drivers/block/drbd/drbd_worker.c @@ -77,10 +77,21 @@ void drbd_md_io_complete(struct bio *bio, int error) md_io->error = error; + /* We grabbed an extra reference in _drbd_md_sync_page_io() to be able + * to timeout on the lower level device, and eventually detach from it. + * If this io completion runs after that timeout expired, this + * drbd_md_put_buffer() may allow us to finally try and re-attach. + * During normal operation, this only puts that extra reference + * down to 1 again. + * Make sure we first drop the reference, and only then signal + * completion, or we may (in drbd_al_read_log()) cycle so fast into the + * next drbd_md_sync_page_io(), that we trigger the + * ASSERT(atomic_read(&mdev->md_io_in_use) == 1) there. + */ + drbd_md_put_buffer(mdev); md_io->done = 1; wake_up(&mdev->misc_wait); bio_put(bio); - drbd_md_put_buffer(mdev); put_ldev(mdev); }