[PATCH -rt] printk: Disable migration instead of preemption

[PATCH -rt] printk: Disable migration instead of preemption

[Richard Weinberger]

There is no need do disable preemption in vprintk(),
disable_migrate() is sufficient.
This fixes also a the following bug in -rt:

[   14.759233] BUG: sleeping function called from invalid context
at /home/rw/linux-rt/kernel/rtmutex.c:645
[   14.759235] in_atomic(): 1, irqs_disabled(): 0, pid: 547, name: bash
[   14.759244] Pid: 547, comm: bash Not tainted 3.0.12-rt29+ #3
[   14.759246] Call Trace:
[   14.759301]  [<ffffffff8106fade>] __might_sleep+0xeb/0xf0
[   14.759318]  [<ffffffff810ad784>] rt_spin_lock_fastlock.constprop.9+0x21/0x43
[   14.759336]  [<ffffffff8161fef0>] rt_spin_lock+0xe/0x10
[   14.759354]  [<ffffffff81347ad1>] serial8250_console_write+0x81/0x121
[   14.759366]  [<ffffffff8107ecd3>] __call_console_drivers+0x7c/0x93
[   14.759369]  [<ffffffff8107ef31>] _call_console_drivers+0x5c/0x60
[   14.759372]  [<ffffffff8107f7e5>] console_unlock+0x147/0x1a2
[   14.759374]  [<ffffffff8107fd33>] vprintk+0x3ea/0x462
[   14.759383]  [<ffffffff816160e0>] printk+0x51/0x53
[   14.759399]  [<ffffffff811974e4>] ? proc_reg_poll+0x9a/0x9a
[   14.759403]  [<ffffffff81335b42>] __handle_sysrq+0x50/0x14d
[   14.759406]  [<ffffffff81335c8a>] write_sysrq_trigger+0x4b/0x53
[   14.759408]  [<ffffffff81335c3f>] ? __handle_sysrq+0x14d/0x14d
[   14.759410]  [<ffffffff81197583>] proc_reg_write+0x9f/0xbe
[   14.759426]  [<ffffffff811497ec>] vfs_write+0xac/0xf3
[   14.759429]  [<ffffffff8114a9b3>] ? fget_light+0x3a/0x9b
[   14.759431]  [<ffffffff811499db>] sys_write+0x4a/0x6e
[   14.759438]  [<ffffffff81625d52>] system_call_fastpath+0x16/0x1b

Signed-off-by: Richard Weinberger <rw@linutronix.de>
---
 kernel/printk.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/printk.c b/kernel/printk.c
index a50af4e..13d0825 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -898,7 +898,7 @@ asmlinkage int vprintk(const char *fmt, va_list args)
 	boot_delay_msec();
 	printk_delay();
 
-	preempt_disable();
+	migrate_disable();
 	/* This stops the holder of console_sem just where we want him */
 	raw_local_irq_save(flags);
 	this_cpu = smp_processor_id();
@@ -1029,7 +1029,7 @@ asmlinkage int vprintk(const char *fmt, va_list args)
 out_restore_irqs:
 	raw_local_irq_restore(flags);
 
-	preempt_enable();
+	migrate_enable();
 	return printed_len;
 }
 EXPORT_SYMBOL(printk);
-- 
1.7.7

Re: [PATCH -rt] printk: Disable migration instead of preemption

[Peter Zijlstra]

On Mon, 2011-12-12 at 14:35 +0100, Richard Weinberger wrote:
> There is no need do disable preemption in vprintk(),
> disable_migrate() is sufficient.
> This fixes also a the following bug in -rt:
> 
> [   14.759233] BUG: sleeping function called from invalid context
> at /home/rw/linux-rt/kernel/rtmutex.c:645

> ---
>  kernel/printk.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/printk.c b/kernel/printk.c
> index a50af4e..13d0825 100644
> --- a/kernel/printk.c
> +++ b/kernel/printk.c
> @@ -898,7 +898,7 @@ asmlinkage int vprintk(const char *fmt, va_list args)
>  	boot_delay_msec();
>  	printk_delay();
>  
> -	preempt_disable();
> +	migrate_disable();
>  	/* This stops the holder of console_sem just where we want him */
>  	raw_local_irq_save(flags);
>  	this_cpu = smp_processor_id();
> @@ -1029,7 +1029,7 @@ asmlinkage int vprintk(const char *fmt, va_list args)
>  out_restore_irqs:
>  	raw_local_irq_restore(flags);
>  
> -	preempt_enable();
> +	migrate_enable();
>  	return printed_len;
>  }
>  EXPORT_SYMBOL(printk);

One has to wonder why this patch makes any difference what so ever, note
how right after/before the preempt_disable/enable there's a
raw_local_irq_save/restore and last time I checked those really did
disable IRQs, even on -rt.

Re: [PATCH -rt] printk: Disable migration instead of preemption

[Thomas Gleixner]

On Mon, 12 Dec 2011, Peter Zijlstra wrote:

> On Mon, 2011-12-12 at 14:35 +0100, Richard Weinberger wrote:
> > There is no need do disable preemption in vprintk(),
> > disable_migrate() is sufficient.
> > This fixes also a the following bug in -rt:
> > 
> > [   14.759233] BUG: sleeping function called from invalid context
> > at /home/rw/linux-rt/kernel/rtmutex.c:645
> 
> > ---
> >  kernel/printk.c |    4 ++--
> >  1 files changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/printk.c b/kernel/printk.c
> > index a50af4e..13d0825 100644
> > --- a/kernel/printk.c
> > +++ b/kernel/printk.c
> > @@ -898,7 +898,7 @@ asmlinkage int vprintk(const char *fmt, va_list args)
> >  	boot_delay_msec();
> >  	printk_delay();
> >  
> > -	preempt_disable();
> > +	migrate_disable();
> >  	/* This stops the holder of console_sem just where we want him */
> >  	raw_local_irq_save(flags);
> >  	this_cpu = smp_processor_id();
> > @@ -1029,7 +1029,7 @@ asmlinkage int vprintk(const char *fmt, va_list args)
> >  out_restore_irqs:
> >  	raw_local_irq_restore(flags);
> >  
> > -	preempt_enable();
> > +	migrate_enable();
> >  	return printed_len;
> >  }
> >  EXPORT_SYMBOL(printk);
> 
> One has to wonder why this patch makes any difference what so ever, note
> how right after/before the preempt_disable/enable there's a
> raw_local_irq_save/restore and last time I checked those really did
> disable IRQs, even on -rt.

The point is that we drop that lock further down and on RT we also
reenable interrupts, so we have to prevent that we get migrated away.

Thanks,

	tglx

Re: [PATCH -rt] printk: Disable migration instead of preemption

[Peter Zijlstra]

On Mon, 2011-12-12 at 17:35 +0100, Thomas Gleixner wrote:
> 
> The point is that we drop that lock further down and on RT we also
> reenable interrupts, so we have to prevent that we get migrated away.
> 
Oh ok. I missed -rt reenabling interrupts somewhere down there.

Do note that these preempt_disable/enable calls are gone upstream.