linux-kernel - [RFC PATCH 1/9] ublk_drv: check 'current' instead of 'ubq

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Message-Id: <20220824054744.77812-2-ZiyangZhang@linux.alibaba.com>
Date:   Wed, 24 Aug 2022 13:47:36 +0800
From:   ZiyangZhang <ZiyangZhang@...ux.alibaba.com>
To:     ming.lei@...hat.com, axboe@...nel.dk
Cc:     xiaoguang.wang@...ux.alibaba.com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, joseph.qi@...ux.alibaba.com,
        ZiyangZhang <ZiyangZhang@...ux.alibaba.com>
Subject: [RFC PATCH 1/9] ublk_drv: check 'current' instead of 'ubq_daemon'

This check is not atomic. So with recovery feature, ubq_daemon may be
updated simultaneously by recovery task. Instead, check 'current' is
safe here because 'current' never changes.

Also add comment explaining this check, which is really important for
understanding recovery feature.

Signed-off-by: ZiyangZhang <ZiyangZhang@...ux.alibaba.com>
---
 drivers/block/ublk_drv.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index 6a4a94b4cdf4..c39b67d7133d 100644
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -645,14 +645,22 @@ static inline void __ublk_rq_task_work(struct request *req)
 	struct ublk_device *ub = ubq->dev;
 	int tag = req->tag;
 	struct ublk_io *io = &ubq->ios[tag];
-	bool task_exiting = current != ubq->ubq_daemon || ubq_daemon_is_dying(ubq);
 	unsigned int mapped_bytes;
 
 	pr_devel("%s: complete: op %d, qid %d tag %d io_flags %x addr %llx\n",
 			__func__, io->cmd->cmd_op, ubq->q_id, req->tag, io->flags,
 			ublk_get_iod(ubq, req->tag)->addr);
 
-	if (unlikely(task_exiting)) {
+	/*
+	 * Task is exiting if either:
+	 *
+	 * (1) current != ubq_daemon.
+	 * io_uring_cmd_complete_in_task() tries to run task_work
+	 * in a workqueue if ubq_daemon(cmd's task) is PF_EXITING.
+	 *
+	 * (2) current->flags & PF_EXITING.
+	 */
+	if (unlikely(current != ubq->ubq_daemon || current->flags & PF_EXITING)) {
 		blk_mq_end_request(req, BLK_STS_IOERR);
 		mod_delayed_work(system_wq, &ub->monitor_work, 0);
 		return;
-- 
2.27.0