Messages in this thread | | | Subject | Re: [v3 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct | From | Yang Shi <> | Date | Tue, 10 Apr 2018 11:28:13 -0700 |
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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 > >
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