From: "Paul E. McKenney" <paulmck@kernel.org>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: Steven Rostedt <rostedt@goodmis.org>,
rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com,
dipankar@in.ibm.com, akpm@linux-foundation.org,
mathieu.desnoyers@efficios.com, josh@joshtriplett.org,
tglx@linutronix.de, peterz@infradead.org, dhowells@redhat.com,
edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH tip/core/rcu 06/30] rcu: Add WRITE_ONCE to rcu_node ->exp_seq_rq store
Date: Mon, 17 Feb 2020 13:36:55 -0800 [thread overview]
Message-ID: <20200217213655.GX2935@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200217211135.GA207704@google.com>
On Mon, Feb 17, 2020 at 04:11:35PM -0500, Joel Fernandes wrote:
> On Sat, Feb 15, 2020 at 02:58:03AM -0800, Paul E. McKenney wrote:
> > On Fri, Feb 14, 2020 at 10:47:43PM -0500, Steven Rostedt wrote:
> > > On Fri, 14 Feb 2020 15:55:43 -0800
> > > paulmck@kernel.org wrote:
> > >
> > > > From: "Paul E. McKenney" <paulmck@kernel.org>
> > > >
> > > > The rcu_node structure's ->exp_seq_rq field is read locklessly, so
> > > > this commit adds the WRITE_ONCE() to a load in order to provide proper
> > > > documentation and READ_ONCE()/WRITE_ONCE() pairing.
> > > >
> > > > This data race was reported by KCSAN. Not appropriate for backporting
> > > > due to failure being unlikely.
> > > >
> > > > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > > > ---
> > > > kernel/rcu/tree_exp.h | 2 +-
> > > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> > > > index d7e0484..85b009e 100644
> > > > --- a/kernel/rcu/tree_exp.h
> > > > +++ b/kernel/rcu/tree_exp.h
> > > > @@ -314,7 +314,7 @@ static bool exp_funnel_lock(unsigned long s)
> > > > sync_exp_work_done(s));
> > > > return true;
> > > > }
> > > > - rnp->exp_seq_rq = s; /* Followers can wait on us. */
> > > > + WRITE_ONCE(rnp->exp_seq_rq, s); /* Followers can wait on us. */
> > >
> > > Didn't Linus say this is basically bogus?
> > >
> > > Perhaps just using it as documenting that it's read locklessly, but is
> > > it really needed?
> >
> > Yes, Linus explicitly stated that WRITE_ONCE() is not required in
> > this case, but he also said that he was OK with it being there for
> > documentation purposes.
>
> Just to add, PeterZ does approve of WRITE_ONCE() to prevent store-tearing
> where applicable.
>
> And I have reproduced Will's example [1] with the arm64 Clang compiler
> shipping with the latest Android NDK just now. It does break up stores when
> writing zeroes to 64-bit valyes, this is a real problem which WRITE_ONCE()
> resolves. And I've verified GCC on arm64 does break up 64-bit immediate value
> writes without WRITE_ONCE().
That does sounds a bit on the required side! ;-)
Thanx, Paul
> thanks,
>
> - Joel
>
> [1] https://lore.kernel.org/lkml/20190821103200.kpufwtviqhpbuv2n@willie-the-truck/
>
>
> > And within RCU, I -do- need it because I absolutely need to see if a
> > given patch introduced new KCSAN reports. So I need it for the same
> > reason that I need the build to proceed without warnings.
> >
> > Others who are working with less concurrency-intensive code might quite
> > reasonably make other choices, of course. And my setting certain KCSAN
> > config options in my own builds doesn't inconvenience them, so we should
> > all be happy, right? :-)
> >
> > Thanx, Paul
> >
> > > -- Steve
> > >
> > >
> > >
> > > > spin_unlock(&rnp->exp_lock);
> > > > trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level,
> > > > rnp->grplo, rnp->grphi, TPS("nxtlvl"));
> > >
next prev parent reply other threads:[~2020-02-17 21:36 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-14 23:55 [PATCH tip/core/rcu 0/30] Miscellaneous fixes for v5.7 Paul E. McKenney
2020-02-14 23:55 ` [PATCH tip/core/rcu 01/30] nfs: Fix nfs_access_get_cached_rcu() sparse error paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 02/30] rcu: Warn on for_each_leaf_node_cpu_mask() from non-leaf paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 03/30] rcu: Fix exp_funnel_lock()/rcu_exp_wait_wake() datarace paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 04/30] rcu: Provide debug symbols and line numbers in KCSAN runs paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 05/30] rcu: Add WRITE_ONCE() to rcu_node ->qsmask update paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 06/30] rcu: Add WRITE_ONCE to rcu_node ->exp_seq_rq store paulmck
2020-02-15 3:47 ` Steven Rostedt
2020-02-15 10:58 ` Paul E. McKenney
2020-02-17 21:11 ` Joel Fernandes
2020-02-17 21:36 ` Paul E. McKenney [this message]
2020-02-14 23:55 ` [PATCH tip/core/rcu 07/30] rcu: Add READ_ONCE() to rcu_node ->gp_seq paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 08/30] rcu: Add WRITE_ONCE() to rcu_state ->gp_req_activity paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 09/30] rcu: Add WRITE_ONCE() to rcu_node ->qsmaskinitnext paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 10/30] locking/rtmutex: rcu: Add WRITE_ONCE() to rt_mutex ->owner paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 11/30] rcu: Add READ_ONCE() to rcu_segcblist ->tails[] paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 12/30] rcu: *_ONCE() for grace-period progress indicators paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 13/30] rcu: Fix typos in beginning comments paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 14/30] rcu: Add READ_ONCE() to rcu_data ->gpwrap paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 15/30] rcu: Add *_ONCE() to rcu_data ->rcu_forced_tick paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 16/30] rcu: Add *_ONCE() to rcu_node ->boost_kthread_status paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 17/30] timer: Use hlist_unhashed_lockless() in timer_pending() paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 18/30] rcu: Remove dead code from rcu_segcblist_insert_pend_cbs() paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 19/30] rcu: Add WRITE_ONCE() to rcu_state ->gp_start paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 20/30] rcu: Fix rcu_barrier_callback() race condition paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 21/30] rculist: Add brackets around cond argument in __list_check_rcu macro paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 22/30] rcu: Don't flag non-starting GPs before GP kthread is running paulmck
2020-02-15 3:53 ` Steven Rostedt
2020-02-15 11:01 ` Paul E. McKenney
2020-02-15 13:42 ` Paul E. McKenney
2020-02-17 20:25 ` Steven Rostedt
2020-02-17 22:03 ` Paul E. McKenney
2020-02-17 22:21 ` Steven Rostedt
2020-02-17 23:03 ` Paul E. McKenney
2020-02-14 23:56 ` [PATCH tip/core/rcu 23/30] rcu: Add missing annotation for rcu_nocb_bypass_lock() paulmck
2020-02-14 23:56 ` [PATCH tip/core/rcu 24/30] rcu/nocb: Add missing annotation for rcu_nocb_bypass_unlock() paulmck
2020-02-14 23:56 ` [PATCH tip/core/rcu 25/30] rcu: Optimize and protect atomic_cmpxchg() loop paulmck
2020-02-14 23:56 ` [PATCH tip/core/rcu 26/30] rcu: Tighten rcu_lockdep_assert_cblist_protected() check paulmck
2020-02-14 23:56 ` [PATCH tip/core/rcu 27/30] rcu: Make nocb_gp_wait() double-check unexpected-callback warning paulmck
2020-02-14 23:56 ` [PATCH tip/core/rcu 28/30] rcu: Mark rcu_state.ncpus to detect concurrent writes paulmck
2020-02-14 23:56 ` [PATCH tip/core/rcu 29/30] rcu: Mark rcu_state.gp_seq " paulmck
2020-02-14 23:56 ` [PATCH tip/core/rcu 30/30] rcu: Make rcu_barrier() account for offline no-CBs CPUs paulmck
2020-02-25 10:24 ` Boqun Feng
2020-02-26 3:14 ` Paul E. McKenney
2020-02-26 4:18 ` Paul E. McKenney
2020-02-26 6:14 ` Boqun Feng
2020-02-26 15:02 ` Paul E. McKenney
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