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

[Paul E. McKenney]

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@xxxxxxxxxx>

Good catch, queued for review and testing.

							Thanx, Paul

> Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: rcu@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  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
>