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

From: Linus Torvalds (torvalds@transmeta.com)
Date: Mon May 07 2001 - 11:51:16 EST

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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This archive was generated by hypermail 2b29 : Mon May 07 2001 - 21:00:25 EST