On 4/10/18 9:21 AM, Yang Shi wrote:
On 4/10/18 5:28 AM, Cyrill Gorcunov wrote:
On Tue, Apr 10, 2018 at 01:10:01PM +0200, Michal Hocko wrote:
Because do_brk does vma manipulations, for this reason it's
running under down_write_killable(&mm->mmap_sem). Or you
mean something else?
Yes, all we need the new lock for is to get a consistent view on brk
values. I am simply asking whether there is something fundamentally
wrong by doing the update inside the new lock while keeping the
original
mmap_sem locking in the brk path. That would allow us to drop the
mmap_sem lock in the proc path when looking at brk values.
Michal gimme some time. I guess we might do so, but I need some
spare time to take more precise look into the code, hopefully today
evening. Also I've a suspicion that we've wracked check_data_rlimit
with this new lock in prctl. Need to verify it again.
I see you guys points. We might be able to move the drop of mmap_sem
before setting mm->brk in sys_brk since mmap_sem should be used to
protect vma manipulation only, then protect the value modify with the
new arg_lock. Then we can eliminate mmap_sem stuff in prctl path, and
it also prevents from wrecking check_data_rlimit.
At the first glance, it looks feasible to me. Will look into deeper
later.
A further look told me this might be *not* feasible.
It looks the new lock will not break check_data_rlimit since in my patch
both start_brk and brk is protected by mmap_sem. The code flow might
look like below:
CPU A CPU B
-------- --------
prctl sys_brk
down_write
check_data_rlimit check_data_rlimit (need mm->start_brk)
set brk
down_write up_write
set start_brk
set brk
up_write
If CPU A gets the mmap_sem first, it will set start_brk and brk, then
CPU B will check with the new start_brk. And, prctl doesn't care if
sys_brk is run before it since it gets the new start_brk and brk from
parameter.
If we protect start_brk and brk with the new lock, sys_brk might get old
start_brk, then sys_brk might break rlimit check silently, is that right?
So, it looks using new lock in prctl and keeping mmap_sem in brk path
has race condition.
Thanks,
Yang
Thanks,
Yang
