On Wed, Jul 31, 2013 at 4:14 PM, Paul Turner <pjt@xxxxxxxxxx> wrote:We attached the following explanatory comment to our version of the patch:
* In the common case (two user threads sharing mm
* switching) the bit will be set; avoid doing a write
* (via atomic test & set) unless we have to. This is
* safe, because no other CPU ever writes to our bit
* in the mask, and interrupts are off (so we can't
* take a TLB IPI here.) If we don't do this, then
* switching threads will pingpong the cpumask
So as mentioned, the "interrupts will be off" is actually dubious.
It's true for the context switch case, but not for the activate_mm().
However, as Rik points out, activate_mm() is different in that we
shouldn't have any preexisting MMU state anyway. And besides, that
should never trigger the "prev == next" case.
But it does look a bit messy, and even your comment is a bit
misleading (it might make somebody think that all of switch_mm() is
protected from interrupts)
Anyway, I'm perfectly ok with the patch itself, but I just wanted to
make sure people had thought about these things.