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[7/7] zfcp: avoid benign overflow of the Request Queue's free-level

Message ID 7f61f59a1f8db270312e64644f9173b8f1ac895f.1593780621.git.bblock@linux.ibm.com
State Mainlined
Commit c3bfffa5ec6901f758160edfb046bdc56d89e8d2
Headers show
Series zfcp: cleanups and small changes for 5.9 | expand

Commit Message

Benjamin Block July 3, 2020, 1:20 p.m. UTC
From: Julian Wiedmann <jwi@linux.ibm.com>

zfcp_qdio_send() and zfcp_qdio_int_req() run concurrently, adding and
completing SBALs on the Request Queue. There's a theoretical race where
zfcp_qdio_int_req() completes a number of SBALs & increments the queue's
free-level _before_ zfcp_qdio_send() was able to decrement it.

This can cause ->req_q_free to momentarily hold a value larger than
QDIO_MAX_BUFFERS_PER_Q. Luckily zfcp_qdio_send() is always called under
->req_q_lock, and all readers of the free-level also take this lock. So
we can trust that zfcp_qdio_send() will clean up such a temporary
overflow before anyone can actually observe it.

But it's still confusing and annoying to worry about. So adjust the code
to avoid this race.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Steffen Maier <maier@linux.ibm.com>
Signed-off-by: Benjamin Block <bblock@linux.ibm.com>
 drivers/s390/scsi/zfcp_qdio.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)
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diff --git a/drivers/s390/scsi/zfcp_qdio.c b/drivers/s390/scsi/zfcp_qdio.c
index d3d110a04884..e78d65bd46b1 100644
--- a/drivers/s390/scsi/zfcp_qdio.c
+++ b/drivers/s390/scsi/zfcp_qdio.c
@@ -260,17 +260,20 @@  int zfcp_qdio_send(struct zfcp_qdio *qdio, struct zfcp_qdio_req *q_req)
+	atomic_sub(sbal_number, &qdio->req_q_free);
 	retval = do_QDIO(qdio->adapter->ccw_device, QDIO_FLAG_SYNC_OUTPUT, 0,
 			 q_req->sbal_first, sbal_number);
 	if (unlikely(retval)) {
+		/* Failed to submit the IO, roll back our modifications. */
+		atomic_add(sbal_number, &qdio->req_q_free);
 		zfcp_qdio_zero_sbals(qdio->req_q, q_req->sbal_first,
 		return retval;
 	/* account for transferred buffers */
-	atomic_sub(sbal_number, &qdio->req_q_free);
 	qdio->req_q_idx += sbal_number;
 	qdio->req_q_idx %= QDIO_MAX_BUFFERS_PER_Q;