Re: Are x86 trap gate handlers safe for preemption?

[Linus Torvalds]

In article <[e-mail]>,
Mikael Pettersson  <[e-mail]> wrote:
>Consider an exception handler like vector 7, device_not_available:
>
>ENTRY(device_not_available)
>        pushl $-1                       # mark this as an int
>        SAVE_ALL
>        movl %cr0, %eax
>        testl $0x4, %eax                # EM (math emulation bit)
>        jne device_not_available_emulate
>        preempt_stop
>
>Since this is invoked via a trap gate and not an interrupt gate,
>what's preventing this code from being preempted and resumed on
>another CPU before the read from %cr0?

Well, since %cr0 should be stable across the task switche, that
shouldn't actually matter.

>				 Another example is the
>machine_check vector (also trap gate) whose handlers access MSRs.

This one looks like a real bug. The fix should be to make it an
interrupt gate, I suspect. Comments?

On the whole, I think it is probably a good idea to make all exceptions
be interrupt gates, and then on a case-by-case basis show why some don't
need to (ie clearly the system calls should _not_ be interrupt gates,
but we've long since made the page fault path use an interrupt gate for
similar special register stability reasons).

		Linus