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

Paul E. McKenney
2024-05-12T17:11:06Z

On Sun, May 12, 2024 at 06:55:29PM +0200, Oleg Nesterov wrote:
> On 05/12, Paul E. McKenney wrote:
> >
> > --- 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;
>
> Thanks Paul!
>
> But then I think this change can also revert this chunk from the previous
> patch:
>
> @@ -122,7 +122,7 @@ void rcu_sync_enter(struct rcu_sync *rsp)
> * we are called at early boot time but this shouldn't happen.
> */
> }
> - rsp->gp_count++;
> + WRITE_ONCE(rsp->gp_count, rsp->gp_count + 1);
> spin_unlock_irq(&rsp->rss_lock);
>
> if (gp_state == GP_IDLE) {

Good catch, thank you! How about like this?

Thanx, Paul

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

commit 3e75ce9876396a770a0fcd8eecd83b9f6469f49c
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..da60a9947c005 100644
--- a/kernel/rcu/sync.c
+++ b/kernel/rcu/sync.c
@@ -122,7 +122,7 @@ void rcu_sync_enter(struct rcu_sync *rsp)
* we are called at early boot time but this shouldn't happen.
*/
}
- WRITE_ONCE(rsp->gp_count, rsp->gp_count + 1);
+ rsp->gp_count++;
spin_unlock_irq(&rsp->rss_lock);

if (gp_state == GP_IDLE) {
@@ -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;