Date:	Fri, 11 Dec 2015 17:03:44 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ani Sinha <ani@...sta.com>
Cc:	Rik van Riel <riel@...hat.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Richard Weinberger <richard@....at>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ivan Delalande <colona@...sta.com>,
	fruggeri <fruggeri@...sta.com>
Subject: Re: new warning on sysrq kernel crash trigger

On Fri, Dec 11, 2015 at 04:16:37PM -0800, Ani Sinha wrote:
> On Fri, 11 Dec 2015, Paul E. McKenney wrote:
> > On Fri, Dec 11, 2015 at 05:10:43PM -0500, Rik van Riel wrote:
> > > On 12/11/2015 03:44 PM, Ani Sinha wrote:
> > > > 
> > > > 
> > > > On Thu, 10 Dec 2015, Paul E. McKenney wrote:
> > > > 
> > > >> On Thu, Dec 10, 2015 at 03:57:09PM -0800, Ani Sinha wrote:
> > > >>> Hi guys
> > > >>>
> > > >>> I am noticing a new warning in linux 3.18 which we did not see before
> > > >>> in linux 3.4 :
> > > >>>
> > > >>> bash-4.1# echo c > /proc/sysrq-trigger
> > > >>> [  978.807185] BUG: sleeping function called from invalid context at
> > > >>> ../arch/x86/mm/fault.c:1187
> > > >>> [  978.909816] in_atomic(): 0, irqs_disabled(): 0, pid: 4706, name: bash
> > > >>> [  978.987358] Preemption disabled at:[<ffffffff81484339>] printk+0x48/0x4a
> > > >>>
> > > >>>
> > > >>> I have bisected this to the following change :
> > > >>>
> > > >>> commit 984d74a72076a12b400339973e8c98fd2fcd90e5
> > > >>> Author: Rik van Riel <riel@...hat.com>
> > > >>> Date:   Fri Jun 6 14:38:13 2014 -0700
> > > >>>
> > > >>>     sysrq: rcu-ify __handle_sysrq
> > > >>>
> > > >>>
> > > >>> the rcu_read_lock() in handle_sysrq() bumps up
> > > >>> current->rcu_read_lock_nesting. Hence, in __do_page_fault() when it
> > > >>> calls might_sleep() in x86/mm/fault.c line 1191,
> > > >>> preempt_count_equals(0) returns false and hence the warning is
> > > >>> printed.
> > > >>>
> > > >>> One way to handle this would be to do something like this:
> > > >>>
> > > >>> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> > > >>> index eef44d9..d4dbe22 100644
> > > >>> --- a/arch/x86/mm/fault.c
> > > >>> +++ b/arch/x86/mm/fault.c
> > > >>> @@ -1132,7 +1132,7 @@ __do_page_fault(struct pt_regs *regs, unsigned
> > > >>> long error_code,
> > > >>>   * If we're in an interrupt, have no user context or are running
> > > >>>   * in a region with pagefaults disabled then we must not take the fault
> > > >>>   */
> > > >>> - if (unlikely(faulthandler_disabled() || !mm)) {
> > > >>> + if (unlikely(faulthandler_disabled() || rcu_preempt_depth() || !mm)) {
> > > >>
> > > >> This works if CONFIG_PREEMPT=y, but if CONFIG_PREEMPT=n, then
> > > >> rcu_preempt_depth() unconditionally returns zero.  And if
> > > >> CONFIG_PREEMPT_COUNT=y && CONFIG_PREEMPT=n, you would still see
> > > >> the might_sleep() splat.
> > > >>
> > > >> Maybe use SRCU instead of RCU for this purpose?
> > > >>
> > > > 
> > > > From ae232ce3fb167b2ad363bfac7aab69001bc55a50 Mon Sep 17 00:00:00 2001
> > > > From: Ani Sinha <ani@...sta.com>
> > > > Date: Fri, 11 Dec 2015 12:07:42 -0800
> > > > Subject: [PATCH 1/1] Fix 'sleeping function called from invalid context'
> > > >  warning in sysrq generated crash.
> > > > 
> > > > Commit 984d74a72076a1 ("sysrq: rcu-ify __handle_sysrq") 
> > > > replaced spin_lock_irqsave() calls with
> > > > rcu_read_lock() calls in sysrq. Since rcu_read_lock() does not
> > > > disable preemption, faulthandler_disabled() in
> > > > __do_page_fault() in x86/fault.c returns false. When the code
> > > > later calls might_sleep() in the pagefault handler, we get the
> > > > following warning:
> > > > 
> > > > BUG: sleeping function called from invalid context at ../arch/x86/mm/fault.c:1187
> > > > in_atomic(): 0, irqs_disabled(): 0, pid: 4706, name: bash
> > > > Preemption disabled at:[<ffffffff81484339>] printk+0x48/0x4a
> > > > 
> > > > To fix this, replace RCU call in handle_sysrq() to use SRCU.
> > > 
> > > The sysrq code can be called from irq context.
> > > 
> > > Trying to use SRCU from an irq context sounds like it could
> > > be a bad idea, though admittedly I do not know enough about
> > > SRCU to know for sure :)
> > 
> > Indeed, not the best idea!  ;-)
> > 
> > I could imagine something like this:
> > 
> > 	if (in_irq())
> > 		rcu_read_lock();
> > 	else
> > 		idx = srcu_read_lock(&sysrq_rcu);
> > 
> > And ditto for unlock.  Then, for the update:
> > 
> > 	synchronize_rcu_mult(call_rcu, call_sysrq_srcu);
> > 
> > Where:
> > 
> > 	static void call_sysrq_srcu(struct rcu_head *head, rcu_callback_t func)
> > 	{
> > 		call_srcu(&sysrq_rcu, head, func);
> > 	}
> > 
> 
> >From ae232ce3fb167b2ad363bfac7aab69001bc55a50 Mon Sep 17 00:00:00 2001
> From: Ani Sinha <ani@...sta.com>
> Date: Fri, 11 Dec 2015 12:07:42 -0800
> Subject: [PATCH 1/1] Fix 'sleeping function called from invalid context'
>  warning in sysrq generated crash.
> 
> Commit 984d74a72076a1 ("sysrq: rcu-ify __handle_sysrq") 
> replaced spin_lock_irqsave() calls with
> rcu_read_lock() calls in sysrq. Since rcu_read_lock() does not
> disable preemption, faulthandler_disabled() in
> __do_page_fault() in x86/fault.c returns false. When the code
> later calls might_sleep() in the pagefault handler, we get the
> following warning:
> 
> BUG: sleeping function called from invalid context at ../arch/x86/mm/fault.c:1187
> in_atomic(): 0, irqs_disabled(): 0, pid: 4706, name: bash
> Preemption disabled at:[<ffffffff81484339>] printk+0x48/0x4a
> 
> To fix this, replace RCU call in handle_sysrq() to use SRCU
> in non-irq context.
> 
> Tested this patch on linux 3.18 by booting off one of our boards.
> 
> Fixes: 984d74a72076a1 ("sysrq: rcu-ify __handle_sysrq")
> 
> Signed-off-by: Ani Sinha <ani@...sta.com>

>From an RCU perspective:

Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

But I must defer to Rik from an sysrq perspective.

							Thanx, Paul

> ---
> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
> index 5381a72..df7d747 100644
> --- a/drivers/tty/sysrq.c
> +++ b/drivers/tty/sysrq.c
> @@ -54,6 +54,7 @@
>  /* Whether we react on sysrq keys or just ignore them */
>  static int __read_mostly sysrq_enabled = CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE;
>  static bool __read_mostly sysrq_always_enabled;
> +DEFINE_STATIC_SRCU(sysrq_rcu);
> 
>  static bool sysrq_on(void)
>  {
> @@ -519,10 +520,13 @@ void __handle_sysrq(int key, bool check_mask)
>  {
>  	struct sysrq_key_op *op_p;
>  	int orig_log_level;
> -	int i;
> +	int i, idx;
> 
>  	rcu_sysrq_start();
> -	rcu_read_lock();
> +	if (in_irq())
> +		rcu_read_lock();
> +	else
> +		idx = srcu_read_lock(&sysrq_rcu);
>  	/*
>  	 * Raise the apparent loglevel to maximum so that the sysrq header
>  	 * is shown to provide the user with positive feedback.  We do not
> @@ -564,7 +568,10 @@ void __handle_sysrq(int key, bool check_mask)
>  		pr_cont("\n");
>  		console_loglevel = orig_log_level;
>  	}
> -	rcu_read_unlock();
> +	if (in_irq())
> +		rcu_read_unlock();
> +	else
> +		srcu_read_unlock(&sysrq_rcu, idx);
>  	rcu_sysrq_end();
>  }
> 
> @@ -1040,6 +1047,11 @@ int sysrq_toggle_support(int enable_mask)
>  	return 0;
>  }
> 
> +static void call_sysrq_srcu(struct rcu_head *head, rcu_callback_t func)
> +{
> +	call_srcu(&sysrq_rcu, head, func);
> +}
> +
>  static int __sysrq_swap_key_ops(int key, struct sysrq_key_op *insert_op_p,
>                                  struct sysrq_key_op *remove_op_p)
>  {
> @@ -1059,7 +1071,7 @@ static int __sysrq_swap_key_ops(int key, struct sysrq_key_op *insert_op_p,
>  	 * Wait for it to go away before returning, so the code for an old
>  	 * op is not freed (eg. on module unload) while it is in use.
>  	 */
> -	synchronize_rcu();
> +	synchronize_rcu_mult(call_rcu, call_sysrq_srcu);
> 
>  	return retval;
>  }
> 

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