Re: [PATCH] x86 page fault handler not interrupt safe

[Linus Torvalds]

On Mon, 7 May 2001, Alan Cox wrote:
> 
> I dont see that we will get a valid value in both cases.
> 
>       get_user
>               fault - set %cr2
>                                       IRQ
>                                       vmalloc
>                                       fault
>                                               set %cr2
>                                               fixup runs
>                                       end IRQ
>               cr2 is corrupt

Wrong. "%cr2" is _not_ "corrupt". It has a well-defined value.

So what happens is

        get_user (or user-mode access)
                fault - set %cr2 to fault1
                        irq
                        vmalloc fault - set %cr2 to fault2
                                fixup runs, iret
                        irq runs, iret
                %cr2 is still %fault2
                vmalloc fault - nothing to do
                "false fixup" runs, iret
        get_user (or user-mode access)
                fault - set %cr2 to fault1
                ... get the right behaviour now ...
                
> There are a whole set of races with the vmalloc fixups.

As far as I can tell, there are no races anywhere. Just silly bugs that
are hard to see.

                Linus

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