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authorJulian Wiedmann <jwi@linux.ibm.com>2020-07-03 15:20:03 +0200
committerMartin K. Petersen <martin.petersen@oracle.com>2020-07-08 00:50:56 -0400
commitc3bfffa5ec6901f758160edfb046bdc56d89e8d2 (patch)
treee2aaadb89761b255669bd3234b1d95f8d6e632b7 /drivers/s390
parent6bcb7c171a0ce0b0114aa088404e552b49d85576 (diff)
scsi: zfcp: Avoid benign overflow of the Request Queue's free-level
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. Link: https://lore.kernel.org/r/7f61f59a1f8db270312e64644f9173b8f1ac895f.1593780621.git.bblock@linux.ibm.com Reviewed-by: Steffen Maier <maier@linux.ibm.com> Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com> Signed-off-by: Benjamin Block <bblock@linux.ibm.com> Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Diffstat (limited to 'drivers/s390')
-rw-r--r--drivers/s390/scsi/zfcp_qdio.c5
1 files changed, 4 insertions, 1 deletions
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;