Date: Sun, 19 May 2013 19:40:09 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>, David Howells <dhowells@redhat.com>,
        Hirokazu Takata <takata@linux-m32r.org>,
        Michal Simek <monstr@monstr.eu>,
        Koichi Yasutake <yasutake.koichi@jp.panasonic.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>, Chris Metcalf <cmetcalf@tilera.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        Arnd Bergmann <arnd@arndb.de>, linux-arm-kernel@lists.infradead.org,
        linux-m32r@ml.linux-m32r.org, linux-m32r-ja@ml.linux-m32r.org,
        microblaze-uclinux@itee.uq.edu.au, linux-am33-list@redhat.com,
        linuxppc-dev@lists.ozlabs.org, linux-arch@vger.kernel.org,
        linux-mm@kvack.org, kvm@vger.kernel.org
Subject: Re: [PATCH v2 10/10] kernel: might_fault does not imply might_sleep
Message-ID: <20130519164009.GA2434@redhat.com>
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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?
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