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@xxxxxxxxxx> > > > > 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@xxxxxxxxxx> > > Cc: Oleg Nesterov <oleg@xxxxxxxxxx> > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx> > > --- > > 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 > > >
