Blaisorblade <[EMAIL PROTECTED]> wrote: > > On Wednesday 21 September 2005 21:49, Andrew Morton wrote: > > "Paolo 'Blaisorblade' Giarrusso" <[EMAIL PROTECTED]> wrote: > > > From: Paolo 'Blaisorblade' Giarrusso <[EMAIL PROTECTED]> > > > 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. ------------------------------------------------------- SF.Net email is sponsored by: Tame your development challenges with Apache's Geronimo App Server. Download it for free - -and be entered to win a 42" plasma tv or your very own Sony(tm)PSP. Click here to play: http://sourceforge.net/geronimo.php _______________________________________________ User-mode-linux-devel mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/user-mode-linux-devel
