Re: [PATCH net] net: fix premature exit from NAPI state polling in napi_disable()

[Alexander Lobakin <alexandr.lobakin@intel.com>]

From: Eric Dumazet <edumazet@google.com>
Date: Wed, 10 Nov 2021 11:24:39 -0800

> On Wed, Nov 10, 2021 at 11:11 AM Alexander Lobakin
> <alexandr.lobakin@intel.com> wrote:
> >
> > Commit 719c57197010 ("net: make napi_disable() symmetric with
> > enable") accidentally introduced a bug sometimes leading to a kernel
> > BUG when bringing an iface up/down under heavy traffic load.
> >
> > Prior to this commit, napi_disable() was polling n->state until
> > none of (NAPIF_STATE_SCHED | NAPIF_STATE_NPSVC) is set and then
> > always flip them. Now there's a possibility to get away with the
> > NAPIF_STATE_SCHE unset as 'continue' drops us to the cmpxchg()
> > call with an unitialized variable, rather than straight to
> > another round of the state check.
> >
> > Error path looks like:
> >
> > napi_disable():
> > unsigned long val, new; /* new is uninitialized */
> >
> > do {
> >         val = READ_ONCE(n->state); /* NAPIF_STATE_NPSVC and/or
> >                                       NAPIF_STATE_SCHED is set */
> >         if (val & (NAPIF_STATE_SCHED | NAPIF_STATE_NPSVC)) { /* true */
> >                 usleep_range(20, 200);
> >                 continue; /* go straight to the condition check */
> >         }
> >         new = val | <...>
> > } while (cmpxchg(&n->state, val, new) != val); /* state == val, cmpxchg()
> >                                                   writes garbage */
> >
> > napi_enable():
> > do {
> >         val = READ_ONCE(n->state);
> >         BUG_ON(!test_bit(NAPI_STATE_SCHED, &val)); /* 50/50 boom */
> > <...>
> >
> > while the typical BUG splat is like:
> >
> > [
> > Fix this by replacing this 'continue' with a goto to the beginning
> > of the loop body to restore the original behaviour.
> > This could be written without a goto, but would look uglier and
> > require one more indent level.
> >
> > Fixes: 719c57197010 ("net: make napi_disable() symmetric with enable")
> > Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
> > Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
> > ---
> >  net/core/dev.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index edeb811c454e..5e101c53b9de 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -6929,10 +6929,11 @@ void napi_disable(struct napi_struct *n)
> >         set_bit(NAPI_STATE_DISABLE, &n->state);
> >
> >         do {
> > +retry:
> >                 val = READ_ONCE(n->state);
> >                 if (val & (NAPIF_STATE_SCHED | NAPIF_STATE_NPSVC)) {
> >                         usleep_range(20, 200);
> > -                       continue;
> > +                       goto retry;
> >                 }
> >
> >                 new = val | NAPIF_STATE_SCHED | NAPIF_STATE_NPSVC;
> > --
> > 2.33.1
> >
> 
> Good catch !

Thanks!

> What about replacing the error prone do {...} while (cmpxchg(..)) by
> something less confusing ?
> 
> This way, no need for a label.
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 5e37d6809317fb3c54686188a908bfcb0bfccdab..9327141892cdaaf0282e082e0c6746abae0f12a7
> 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -6264,7 +6264,7 @@ void napi_disable(struct napi_struct *n)
>         might_sleep();
>         set_bit(NAPI_STATE_DISABLE, &n->state);
> 
> -       do {
> +       for (;;) {
>                 val = READ_ONCE(n->state);
>                 if (val & (NAPIF_STATE_SCHED | NAPIF_STATE_NPSVC)) {
>                         usleep_range(20, 200);
> @@ -6273,7 +6273,9 @@ void napi_disable(struct napi_struct *n)
> 
>                 new = val | NAPIF_STATE_SCHED | NAPIF_STATE_NPSVC;
>                 new &= ~(NAPIF_STATE_THREADED | NAPIF_STATE_PREFER_BUSY_POLL);
> -       } while (cmpxchg(&n->state, val, new) != val);
> +               if (cmpxchg(&n->state, val, new) == val)
> +                       break;
> +       }
> 
>         hrtimer_cancel(&n->timer);

LFTM, I'l queue v2 in a moment with you in Suggested-by.

Thanks,
Al