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

["Paul E. McKenney" <paulmck@xxxxxxxxxx>]

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@xxxxxxxxxx>
> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>

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@xxxxxxxxxx>
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@xxxxxxxxxx>
    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

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;