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

Benjamin Block
2020-07-03T13:21:17Z

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(-)

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)
zfcp_qdio_account(qdio);
spin_unlock(&qdio->stat_lock);

+ 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,
sbal_number);
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;

--
2.26.2