Re: [PATCH 25/48] rcu: Mark writes to rcu_sync ->gp_count field

Paul E. McKenney
2024-05-07T17:44:49Z

On Tue, May 07, 2024 at 10:54:41AM -0400, Oleg Nesterov wrote:
> Hello,
>
> I feel I don't really like this patch but I am travelling without my working
> laptop, can't read the source code ;) Quite possibly I am wrong, I'll return
> to this when I get back on May 10.

By the stricter data-race rules used in RCU code [1], this is a bug that
needs to be fixed. This code is updating ->gp_count, which is read
locklessly, which in turn results in a data race. The fix is to mark
the updates (as below) with WRITE_ONCE().

Or is there something in one or the other of these updates to ->gp_count
that excludes lockless readers? (I am not seeing it, but you know this
code way better than I do!)

Thanx, Paul

[1] https://docs.google.com/document/d/1FwZaXSg3A55ivVoWffA9iMuhJ3_Gmj_E494dLYjjyLQ/edit?usp=sharing

> Oleg.
>
> On 05/07, Uladzislau Rezki (Sony) wrote:
> >
> > From: "Paul E. McKenney" <paulmck@kernel.org>
> >
> > The rcu_sync structure's ->gp_count field is updated under the protection
> > of ->rss_lock, but read locklessly, and KCSAN noted the data race.
> > This commit therefore uses WRITE_ONCE() to do this update to clearly
> > document its racy nature.
> >
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > Cc: Oleg Nesterov <oleg@redhat.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > ---
> > kernel/rcu/sync.c | 8 ++++++--
> > 1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
> > index 86df878a2fee..6c2bd9001adc 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.
> > */
> > }
> > - rsp->gp_count++;
> > + WRITE_ONCE(rsp->gp_count, rsp->gp_count + 1);
> > spin_unlock_irq(&rsp->rss_lock);
> >
> > if (gp_state == GP_IDLE) {
> > @@ -151,11 +151,15 @@ 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);
> > - if (!--rsp->gp_count) {
> > + gpc = rsp->gp_count - 1;
> > + WRITE_ONCE(rsp->gp_count, gpc);
> > + if (!gpc) {
> > if (rsp->gp_state == GP_PASSED) {
> > WRITE_ONCE(rsp->gp_state, GP_EXIT);
> > rcu_sync_call(rsp);
> > --
> > 2.39.2
> >
>