Re: [v4 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[Cyrill Gorcunov]

On Tue, Apr 17, 2018 at 11:29:57AM -0700, Andrew Morton wrote:
> On Sun, 15 Apr 2018 02:24:51 +0800 Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> wrote:
> 
> > mmap_sem is on the hot path of kernel, and it very contended, but it is
> > abused too. It is used to protect arg_start|end and evn_start|end when
> > reading /proc/$PID/cmdline and /proc/$PID/environ, but it doesn't make
> > sense since those proc files just expect to read 4 values atomically and
> > not related to VM, they could be set to arbitrary values by C/R.
> > 
> > And, the mmap_sem contention may cause unexpected issue like below:
> > 
> > INFO: task ps:14018 blocked for more than 120 seconds.
> >        Tainted: G            E 4.9.79-009.ali3000.alios7.x86_64 #1
> >  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
> > message.
> >  ps              D    0 14018      1 0x00000004
> >   ffff885582f84000 ffff885e8682f000 ffff880972943000 ffff885ebf499bc0
> >   ffff8828ee120000 ffffc900349bfca8 ffffffff817154d0 0000000000000040
> >   00ffffff812f872a ffff885ebf499bc0 024000d000948300 ffff880972943000
> >  Call Trace:
> >   [<ffffffff817154d0>] ? __schedule+0x250/0x730
> >   [<ffffffff817159e6>] schedule+0x36/0x80
> >   [<ffffffff81718560>] rwsem_down_read_failed+0xf0/0x150
> >   [<ffffffff81390a28>] call_rwsem_down_read_failed+0x18/0x30
> >   [<ffffffff81717db0>] down_read+0x20/0x40
> >   [<ffffffff812b9439>] proc_pid_cmdline_read+0xd9/0x4e0
> >   [<ffffffff81253c95>] ? do_filp_open+0xa5/0x100
> >   [<ffffffff81241d87>] __vfs_read+0x37/0x150
> >   [<ffffffff812f824b>] ? security_file_permission+0x9b/0xc0
> >   [<ffffffff81242266>] vfs_read+0x96/0x130
> >   [<ffffffff812437b5>] SyS_read+0x55/0xc0
> >   [<ffffffff8171a6da>] entry_SYSCALL_64_fastpath+0x1a/0xc5
> > 
> > Both Alexey Dobriyan and Michal Hocko suggested to use dedicated lock
> > for them to mitigate the abuse of mmap_sem.
> > 
> > So, introduce a new spinlock in mm_struct to protect the concurrent
> > access to arg_start|end, env_start|end and others, as well as replace
> > write map_sem to read to protect the race condition between prctl and
> > sys_brk which might break check_data_rlimit(), and makes prctl more
> > friendly to other VM operations.
> 
> (We should move check_data_rlimit() out of the .h file)
> 
> It seems inconsistent to be using mmap_sem to protect ->start_brk and
> friends in sys_brk().  We've already declared that these are protected
> by arg_lock so that's what we should be using?  And getting this
> consistent should permit us to stop using mmap_sem in prctl()
> altogether?

Nope, we still can't. Look, the down_read part order the call with
sys_brk. while arg_lock orders prctl call itself. That said if
someone is calling sys_brk while we're in a middle of prctl it
should wait until prctl finished. But two simultaneous prcl
may proceed without taking a write lock using arg_lock as
a barrier.