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Date: Wed, 26 Nov 2014 19:21:39 +0200 From: "Michael S. Tsirkin" <mst@...hat.com> To: Christian Borntraeger <borntraeger@...ibm.com> Cc: David Hildenbrand <dahi@...ux.vnet.ibm.com>, linuxppc-dev@...ts.ozlabs.org, linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org, benh@...nel.crashing.org, paulus@...ba.org, akpm@...ux-foundation.org, heiko.carstens@...ibm.com, schwidefsky@...ibm.com, mingo@...nel.org Subject: Re: [RFC 0/2] Reenable might_sleep() checks for might_fault() when atomic On Wed, Nov 26, 2014 at 07:04:47PM +0200, Michael S. Tsirkin wrote: > On Wed, Nov 26, 2014 at 05:51:08PM +0100, Christian Borntraeger wrote: > > > But this one was > giving users in field false positives. > > > > So lets try to fix those, ok? If we cant, then tough luck. > > Sure. > I think the simplest way might be to make spinlock disable > premption when CONFIG_DEBUG_ATOMIC_SLEEP is enabled. Specifically maybe DEBUG_ATOMIC_SLEEP should select PREEMPT_COUNT? > As a result, userspace access will fail and caller will > get a nice error. > > > > > But coming up with wrong statements is not helpful. > > True. Sorry that I did that. > > > > > > > The point is that *_user is safe with preempt off. > > > It returns an error gracefully. > > > It does not sleep. > > > It does not trigger the scheduler in that context. > > > > There are special cases where your statement is true. But its not in general. > > copy_to_user might fault and that fault might sleep and reschedule. > > Yes. But not if called inatomic. > > > > > For example handle_mm_fault might go down to pud_alloc, pmd_alloc etc and all these functions could do an GFP_KERNEL allocation. Which might sleep. Which will schedule. > > > > > > > > > > > > > David's patch makes it say it does, so it's wrong. > > > > > > > > > > > Absolutely. > I think you can already debug your case easily, by enabling CONFIG_PREEMPT. > This seems counter-intuitive, and distro debug kernels don't seem to do this. > > -- > MST -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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