Re: Are x86 trap gate handlers safe for preemption?

Linus Torvalds
2002-10-30T22:45:21Z

In article <15808.17731.311432.596865@kim.it.uu.se>,
Mikael Pettersson <mikpe@csd.uu.se> 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