Re: [PATCH] rcu/sync: don't read rcu_sync->gp_count lockless

Paul E. McKenney
2024-05-12T15:10:56Z

On Sun, May 12, 2024 at 01:19:48PM +0200, Oleg Nesterov wrote:
> rcu_sync->gp_count is updated under the protection of ->rss_lock but read
> locklessly by the WARN_ON() checks, and KCSAN noted the data race.
>
> Move these WARN_ON_ONCE()'s under the lock and remove the no longer needed
> READ_ONCE().
>
> Reported-by: "Paul E. McKenney" <paulmck@kernel.org>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Very good, thank you!

Due to inattention on my part, the patches were sent late, so the patch
you are (rightly) complaining about is on its way in. So what I did was
to port your patch on top of that one as shown below. Left to myself,
I would be thinking in terms of the v6.11 merge window. Please let me
know if this is more urgent than that.

And as always, please let me know if I messed anything on in the port.

Thanx, Paul

------------------------------------------------------------------------

commit 8d75fb302aaa97693c2294ded48a472e4956d615
Author: Oleg Nesterov <oleg@redhat.com>
Date: Sun May 12 08:02:07 2024 -0700

rcu: Eliminate lockless accesses to rcu_sync->gp_count

The rcu_sync structure's ->gp_count field is always accessed under the
protection of that same structure's ->rss_lock field, with the exception
of a pair of WARN_ON_ONCE() calls just prior to acquiring that lock in
functions rcu_sync_exit() and rcu_sync_dtor(). These lockless accesses
are unnecessary and impair KCSAN's ability to catch bugs that might be
inserted via other lockless accesses.

This commit therefore moves those WARN_ON_ONCE() calls under the lock.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
index 6c2bd9001adcd..05bfe69fdb0bb 100644
--- a/kernel/rcu/sync.c
+++ b/kernel/rcu/sync.c
@@ -151,15 +151,11 @@ void rcu_sync_enter(struct rcu_sync *rsp)
*/
void rcu_sync_exit(struct rcu_sync *rsp)
{
- int gpc;
-
WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
- WARN_ON_ONCE(READ_ONCE(rsp->gp_count) == 0);

spin_lock_irq(&rsp->rss_lock);
- gpc = rsp->gp_count - 1;
- WRITE_ONCE(rsp->gp_count, gpc);
- if (!gpc) {
+ WARN_ON_ONCE(rsp->gp_count == 0);
+ if (!--rsp->gp_count) {
if (rsp->gp_state == GP_PASSED) {
WRITE_ONCE(rsp->gp_state, GP_EXIT);
rcu_sync_call(rsp);
@@ -178,10 +174,10 @@ void rcu_sync_dtor(struct rcu_sync *rsp)
{
int gp_state;

- WARN_ON_ONCE(READ_ONCE(rsp->gp_count));
WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED);

spin_lock_irq(&rsp->rss_lock);
+ WARN_ON_ONCE(rsp->gp_count);
if (rsp->gp_state == GP_REPLAY)
WRITE_ONCE(rsp->gp_state, GP_EXIT);
gp_state = rsp->gp_state;