[PATCH net] rds: Make sure updates to cp_send_gen can be observed

Håkon Bugge
2017-07-20T10:28:55Z

cp->cp_send_gen is treated as a normal variable, although it may be
used by different threads.

This is fixed by using {READ,WRITE}_ONCE when it is incremented and
READ_ONCE when it is read outside the {acquire,release}_in_xmit
protection.

Normative reference from the Linux-Kernel Memory Model:

Loads from and stores to shared (but non-atomic) variables should
be protected with the READ_ONCE(), WRITE_ONCE(), and
ACCESS_ONCE().

Clause 5.1.2.4/25 in the C standard is also relevant.

Signed-off-by: Håkon Bugge <haakon.bugge@oracle.com>
Reviewed-by: Knut Omang <knut.omang@oracle.com>
---
net/rds/send.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/rds/send.c b/net/rds/send.c
index 5cc6403..fa0368c 100644
--- a/net/rds/send.c
+++ b/net/rds/send.c
@@ -170,8 +170,8 @@ int rds_send_xmit(struct rds_conn_path *cp)
* The acquire_in_xmit() check above ensures that only one
* caller can increment c_send_gen at any time.
*/
- cp->cp_send_gen++;
- send_gen = cp->cp_send_gen;
+ send_gen = READ_ONCE(cp->cp_send_gen) + 1;
+ WRITE_ONCE(cp->cp_send_gen, send_gen);

/*
* rds_conn_shutdown() sets the conn state and then tests RDS_IN_XMIT,
@@ -431,7 +431,7 @@ int rds_send_xmit(struct rds_conn_path *cp)
smp_mb();
if ((test_bit(0, &conn->c_map_queued) ||
!list_empty(&cp->cp_send_queue)) &&
- send_gen == cp->cp_send_gen) {
+ send_gen == READ_ONCE(cp->cp_send_gen)) {
rds_stats_inc(s_send_lock_queue_raced);
if (batch_count < send_batch_count)
goto restart;
--
2.9.3