Subject: Re: [PATCH v2 10/10] kernel: might_fault does not imply might_sleep

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 19 May 2013 19:40:09 +0300

On Sun, May 19, 2013 at 12:06:19PM -0400, Steven Rostedt wrote:
> On Sun, 2013-05-19 at 16:34 +0300, Michael S. Tsirkin wrote:
> 
> > Right but we need to keep it working on upstream as well.
> > If I do preempt_enable under a spinlock upstream won't it
> > try to sleep under spinlock?
> 
> No it wont. A spinlock calls preempt_disable implicitly, and a
> preempt_enable() will not schedule unless preempt_count is zero, which
> it wont be under a spinlock.
> 
> If it did, there would be lots of bugs all over the place because this
> is done throughout the kernel (a preempt_enable() under a spinlock).
> 
> In other words, don't ever use preempt_enable_no_resched().
> 
> -- Steve
> 

OK I get it. So let me correct myself. The simple code
that does something like this under a spinlock:
>       preempt_disable
>       pagefault_disable
>       error = copy_to_user
>       pagefault_enable
>       preempt_enable
is not doing anything wrong and should not get a warning,
as long as error is handled correctly later.
Right?