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

[Peter Zijlstra <peterz@infradead.org>]

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.