Re: new warning on sysrq kernel crash trigger

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Dec 11, 2015 at 12:44:13PM -0800, 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@redhat.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@arista.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.
> 
> 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@arista.com>

Hello, Ani,

This patch looks incomplete.  The synchronize_rcu() that Rik added in
__sysrq_swap_key_ops() needs to become synchronize_srcu().  Which
means that it needs to use the sysrq_rcu structure, which means
that this structure cannot be local to __handle_sysrq().

Please see below...

> ---
>  drivers/tty/sysrq.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
> index 5381a72..904865f 100644
> --- a/drivers/tty/sysrq.c
> +++ b/drivers/tty/sysrq.c
> @@ -519,10 +519,12 @@ void __handle_sysrq(int key, bool check_mask)
>  {
>  	struct sysrq_key_op *op_p;
>  	int orig_log_level;
> -	int i;
> +	int i, idx;
> +	struct srcu_struct sysrq_rcu;
> 
> +	init_srcu_struct(&sysrq_rcu);

Use DEFINE_STATIC_SRCU() to define sysrq_rcu at the global level,
and then get rid of the two lines above.

							Thanx, Paul

>  	rcu_sysrq_start();
> -	rcu_read_lock();
> +	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 +566,7 @@ void __handle_sysrq(int key, bool check_mask)
>  		pr_cont("\n");
>  		console_loglevel = orig_log_level;
>  	}
> -	rcu_read_unlock();
> +	srcu_read_unlock(&sysrq_rcu, idx);
>  	rcu_sysrq_end();
>  }
> 
> -- 
> 1.8.1.4
>