Messages in this thread | | | Date | Wed, 21 Sep 2005 13:47:24 -0700 | From | Andrew Morton <> | Subject | Re: [uml-devel] Re: [PATCH 07/10] uml: avoid fixing faults while atomic |
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Blaisorblade <blaisorblade@yahoo.it> wrote: > > On Wednesday 21 September 2005 21:49, Andrew Morton wrote: > > "Paolo 'Blaisorblade' Giarrusso" <blaisorblade@yahoo.it> wrote: > > > From: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it> > > > The in_atomic() test in x86's do_page_fault() is in fact a message passed > > into it from filemap.c's kmap_atomic(). > Ok, this can be ok, but: > > It has accidental side-effects, > > such as making copy_to_user() fail if inside spinlocks when > > CONFIG_PREEMPT=y. > Sorry, but should it ever succeed inside spinlocks? I mean, should it ever > call down() inside spinlocks? (We never do down_trylock, and ever if we did > the x86 trick, that wouldn't make the whole thing safe at all - they still > take the spinlock and potentially sleep. And it's legal only if no spinlock > is held).
Not sure what you're asking here.
copy_to/from_user() will fail inside spinlock if CONFIG_PREMPT=y and if the copy happens to cause a fault. Otherwise it will succeed inside spinlock, and it won't spew a sleeping-while-atomic warning, because that uses in_atomic() too. It might deadlock if we schedule away and try to retake the same lock.
> Even if spinlocks don't always trigger in_atomic() - which means that we'd > need to have a better fix for this.
The patch you have will correctly cause copy_*_user()->pagefault to fail the copy if the caller has run inc_preempt_count(). It will not cause copy_*_user()->pagefault to fail inside spinlocks unless UML does inc_preempt_count() in its spinlock implementation.
> (Btw, I took the above reasoning from something said, as an aside, on LWN.net > kernel page, about the FUTEX deadlock on mm->mmap_sem of ~ 2.6.8 - yes, it > wasn't the full truth, but not totally dumb). > > > So I think this change is only needed if UML implements kmap_atomic, as in > > arch/i386/mm/highmem.c, which it surely does not do? > NACK, see above.
Yup, the patch is needed for the futex code, and for general correct implementation of inc_preempt_count()'s intended effect.
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