Re: [PATCH] rcu: Fix data-race due to atomic_t copy-by-value

[Marco Elver <elver@google.com>]

On Mon, 14 Oct 2019 at 20:00, Joel Fernandes <joel@joelfernandes.org> wrote:
>
> On Wed, Oct 09, 2019 at 05:57:43PM +0200, Marco Elver wrote:
> > This fixes a data-race where `atomic_t dynticks` is copied by value. The
> > copy is performed non-atomically, resulting in a data-race if `dynticks`
> > is updated concurrently.
> >
> > This data-race was found with KCSAN:
> > ==================================================================
> > BUG: KCSAN: data-race in dyntick_save_progress_counter / rcu_irq_enter
> >
> > write to 0xffff989dbdbe98e0 of 4 bytes by task 10 on cpu 3:
> >  atomic_add_return include/asm-generic/atomic-instrumented.h:78 [inline]
> >  rcu_dynticks_snap kernel/rcu/tree.c:310 [inline]
> >  dyntick_save_progress_counter+0x43/0x1b0 kernel/rcu/tree.c:984
> >  force_qs_rnp+0x183/0x200 kernel/rcu/tree.c:2286
> >  rcu_gp_fqs kernel/rcu/tree.c:1601 [inline]
> >  rcu_gp_fqs_loop+0x71/0x880 kernel/rcu/tree.c:1653
> >  rcu_gp_kthread+0x22c/0x3b0 kernel/rcu/tree.c:1799
> >  kthread+0x1b5/0x200 kernel/kthread.c:255
> >  <snip>
> >
> > read to 0xffff989dbdbe98e0 of 4 bytes by task 154 on cpu 7:
> >  rcu_nmi_enter_common kernel/rcu/tree.c:828 [inline]
> >  rcu_irq_enter+0xda/0x240 kernel/rcu/tree.c:870
> >  irq_enter+0x5/0x50 kernel/softirq.c:347
> >  <snip>
> >
> > Reported by Kernel Concurrency Sanitizer on:
> > CPU: 7 PID: 154 Comm: kworker/7:1H Not tainted 5.3.0+ #5
> > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
> > Workqueue: kblockd blk_mq_run_work_fn
> > ==================================================================
> >
> > Signed-off-by: Marco Elver <elver@google.com>
>
> I believe Paul has already queued this, but anyway I have reviewed it as well
> and it LGTM.
>
> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

Thank you! As an aside, I was wondering if there is a way to disallow
copy-by-value for certain structs, such as atomic_t (in the absence of
constructors). Presumably that'd be useful to have here to just let
the type-system prevent similar cases in future.

Thanks,
-- Marco

> thanks,
>
>  - Joel
>
>
> > ---
> >  include/trace/events/rcu.h |  4 ++--
> >  kernel/rcu/tree.c          | 11 ++++++-----
> >  2 files changed, 8 insertions(+), 7 deletions(-)
> >
> > diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
> > index 694bd040cf51..fdd31c5fd126 100644
> > --- a/include/trace/events/rcu.h
> > +++ b/include/trace/events/rcu.h
> > @@ -442,7 +442,7 @@ TRACE_EVENT_RCU(rcu_fqs,
> >   */
> >  TRACE_EVENT_RCU(rcu_dyntick,
> >
> > -     TP_PROTO(const char *polarity, long oldnesting, long newnesting, atomic_t dynticks),
> > +     TP_PROTO(const char *polarity, long oldnesting, long newnesting, int dynticks),
> >
> >       TP_ARGS(polarity, oldnesting, newnesting, dynticks),
> >
> > @@ -457,7 +457,7 @@ TRACE_EVENT_RCU(rcu_dyntick,
> >               __entry->polarity = polarity;
> >               __entry->oldnesting = oldnesting;
> >               __entry->newnesting = newnesting;
> > -             __entry->dynticks = atomic_read(&dynticks);
> > +             __entry->dynticks = dynticks;
> >       ),
> >
> >       TP_printk("%s %lx %lx %#3x", __entry->polarity,
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 81105141b6a8..62e59596a30a 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -576,7 +576,7 @@ static void rcu_eqs_enter(bool user)
> >       }
> >
> >       lockdep_assert_irqs_disabled();
> > -     trace_rcu_dyntick(TPS("Start"), rdp->dynticks_nesting, 0, rdp->dynticks);
> > +     trace_rcu_dyntick(TPS("Start"), rdp->dynticks_nesting, 0, atomic_read(&rdp->dynticks));
> >       WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
> >       rdp = this_cpu_ptr(&rcu_data);
> >       do_nocb_deferred_wakeup(rdp);
> > @@ -649,14 +649,15 @@ static __always_inline void rcu_nmi_exit_common(bool irq)
> >        * leave it in non-RCU-idle state.
> >        */
> >       if (rdp->dynticks_nmi_nesting != 1) {
> > -             trace_rcu_dyntick(TPS("--="), rdp->dynticks_nmi_nesting, rdp->dynticks_nmi_nesting - 2, rdp->dynticks);
> > +             trace_rcu_dyntick(TPS("--="), rdp->dynticks_nmi_nesting, rdp->dynticks_nmi_nesting - 2,
> > +                               atomic_read(&rdp->dynticks));
> >               WRITE_ONCE(rdp->dynticks_nmi_nesting, /* No store tearing. */
> >                          rdp->dynticks_nmi_nesting - 2);
> >               return;
> >       }
> >
> >       /* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */
> > -     trace_rcu_dyntick(TPS("Startirq"), rdp->dynticks_nmi_nesting, 0, rdp->dynticks);
> > +     trace_rcu_dyntick(TPS("Startirq"), rdp->dynticks_nmi_nesting, 0, atomic_read(&rdp->dynticks));
> >       WRITE_ONCE(rdp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */
> >
> >       if (irq)
> > @@ -743,7 +744,7 @@ static void rcu_eqs_exit(bool user)
> >       rcu_dynticks_task_exit();
> >       rcu_dynticks_eqs_exit();
> >       rcu_cleanup_after_idle();
> > -     trace_rcu_dyntick(TPS("End"), rdp->dynticks_nesting, 1, rdp->dynticks);
> > +     trace_rcu_dyntick(TPS("End"), rdp->dynticks_nesting, 1, atomic_read(&rdp->dynticks));
> >       WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
> >       WRITE_ONCE(rdp->dynticks_nesting, 1);
> >       WARN_ON_ONCE(rdp->dynticks_nmi_nesting);
> > @@ -827,7 +828,7 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
> >       }
> >       trace_rcu_dyntick(incby == 1 ? TPS("Endirq") : TPS("++="),
> >                         rdp->dynticks_nmi_nesting,
> > -                       rdp->dynticks_nmi_nesting + incby, rdp->dynticks);
> > +                       rdp->dynticks_nmi_nesting + incby, atomic_read(&rdp->dynticks));
> >       WRITE_ONCE(rdp->dynticks_nmi_nesting, /* Prevent store tearing. */
> >                  rdp->dynticks_nmi_nesting + incby);
> >       barrier();
> > --
> > 2.23.0.581.g78d2f28ef7-goog
> >
> >