Date: Thu, 7 May 2015 11:48:19 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <dahi@linux.vnet.ibm.com>, linux-kernel@vger.kernel.org,
        mingo@redhat.com, peterz@infradead.org, yang.shi@windriver.com,
        bigeasy@linutronix.de, benh@kernel.crashing.org, paulus@samba.org,
        heiko.carstens@de.ibm.com, schwidefsky@de.ibm.com,
        borntraeger@de.ibm.com, mst@redhat.com, tglx@linutronix.de,
        David.Laight@ACULAB.COM, hughd@google.com, hocko@suse.cz,
        ralf@linux-mips.org, herbert@gondor.apana.org.au,
        linux@arm.linux.org.uk, airlied@linux.ie, daniel.vetter@intel.com,
        linux-mm@kvack.org, linux-arch@vger.kernel.org
Subject: Re: [PATCH RFC 00/15] decouple pagefault_disable() from
 preempt_disable()
Message-ID: <20150507094819.GC4734@gmail.com>
References: <1430934639-2131-1-git-send-email-dahi@linux.vnet.ibm.com>
 <20150506150158.0a927470007e8ea5f3278956@linux-foundation.org>
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* Andrew Morton <akpm@linux-foundation.org> wrote:

> On Wed,  6 May 2015 19:50:24 +0200 David Hildenbrand <dahi@linux.vnet.ibm.com> wrote:
> 
> > As Peter asked me to also do the decoupling in one shot, this is
> > the new series.
> > 
> > I recently discovered that might_fault() doesn't call might_sleep()
> > anymore. Therefore bugs like:
> > 
> >   spin_lock(&lock);
> >   rc = copy_to_user(...);
> >   spin_unlock(&lock);
> > 
> > would not be detected with CONFIG_DEBUG_ATOMIC_SLEEP. The code was
> > changed to disable false positives for code like:
> > 
> >   pagefault_disable();
> >   rc = copy_to_user(...);
> >   pagefault_enable();
> > 
> > Whereby the caller wants do deal with failures.
> 
> hm, that was a significant screwup.  I wonder how many bugs we
> subsequently added.

So I'm wondering what the motivation was to allow things like:

   pagefault_disable();
   rc = copy_to_user(...);
   pagefault_enable();

and to declare it a false positive?

AFAICS most uses are indeed atomic:

        pagefault_disable();
        ret = futex_atomic_cmpxchg_inatomic(curval, uaddr, uval, newval);
        pagefault_enable();

so why not make it explicitly atomic again?

Thanks,

	Ingo
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