On 07/11/2016 12:21 PM, James Hogan wrote:
On Mon, Jul 11, 2016 at 11:19:30AM -0700, Leonid Yegoshin wrote:
On 07/11/2016 11:07 AM, James Hogan wrote:
Not exactly. The change must be done only for local CPU which executes
at the moment get_new_mmu_context(). Just prevent preemption here and
change of cpu_context(THIS_CPU,...) can be done safely - other CPUs
don't do anything with this variable besides killing it (writing 0 to it).
Right, but I was thinking more along the lines of whether you can ensure
the other tasks / mm continues to exist. I think this is partly achieved
by the read_lock'ing of tasklist_lock, but also possibly by the
find_lock_task_mm() call, which has a comment saying:
/*
* The process p may have detached its own ->mm while exiting or through
* use_mm(), but one or more of its subthreads may still have a valid
* pointer. Return p, or any of its subthreads with a valid ->mm, with
* task_lock() held.
*/
(but of course I could be mistaken and something else guarantees it
won't go away).
I don't look into details of that but a safe way to do is - walk through
all memory maps and lock it before change.
And to walk through memory maps we could use something like
'find_lock_task_mm' but if there is a concern like you stated above then
we could walk through all subthreads of task or just through all threads
in system - anywhere, this even (ASID wrap) is pretty rare.
The advantage is in keeping all that stuff local and avoid patching
other arch and common code.
Note also that I have a patch I'm about to submit which changes some of
those assignments of 0 to assign 1 instead (so as not to confuse the
cache management code into thinking the CPU has never run the code when
it has, while still triggering ASID regeneration). That applies here
too, so it should perhaps be doing something like this instead:
if (t->mm != mm && cpu_context(cpu, t->mm))
cpu_context(cpu, t->mm) = 1;
Not sure, but did you have chance to look into having another variable
for cache flush control? It can be that some more states may be needed
in future, so - just disjoin both, TLB and cache coontrol.
- Leonid.
Cheers
James
You can look into flush_tlb_mm() for example how it is cleared for
single memory map.
We have a macro to safely walk all processes, right? (don't remember
it's name).
