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 * cacheline. */
On Wed, Jul 31, 2013 at 4:12 PM, Rik van Riel <[email protected]> wrote: > On 07/31/2013 06:46 PM, Linus Torvalds wrote: >> >> >> On Jul 31, 2013 3:39 PM, "Rik van Riel" <[email protected] >> >> <mailto:[email protected]>> wrote: >> > >> > On 07/31/2013 06:21 PM, Linus Torvalds wrote: >> >> >> >> Ummm.. The race is to the testing of the bit, not setting. The testing >> >> of the bit is not valid before we have set the tlb state, AFAIK. >> > >> > >> > I believe the bit is cleared and set by the current CPU. >> >> Yes, but we need to be careful with interrupts. >> >> >> > Interrupts are blocked inside switch_mm, so I think we >> > are safe. >> >> Are they? I thought we removed all that. > > > context_switch() shows that the runqueue lock (which is an irq > lock) is released, and irqs re-enabled, by the next task, after > switch_to(), in finish_lock_switch(), called from finish_task_switch() > >> Note that switch_mm gets called for activate_mm too, or something. > > > Good catch, though it looks like activate_mm is only called from > exec_mmap, with the new mm as its argument. While the new mm can > have pages in memory yet, it has never been run so there should > be nothing in the TLB yet for the new mm. > > This is subtler than I thought, but it does appear to be safe. > > > -- > All rights reversed > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to [email protected] > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
