Re: [PATCH] blk-throttle: Use task_subsys_state() to determine a task's blkio_cgroup

[Li Zefan <lizf@cn.fujitsu.com>]

Vivek Goyal wrote:
> Currentlly we first map the task to cgroup and then cgroup to
> blkio_cgroup. There is a more direct way to get to blkio_cgroup
> from task using task_subsys_state(). Use that.
> 
> The real reason for the fix is that it also avoids a race in generic
> cgroup code. During remount/umount rebind_subsystems() is called and
> it can do following with and rcu protection.
> 
> cgrp->subsys[i] = NULL;
> 
> That means if somebody got hold of cgroup under rcu and then it tried
> to do cgroup->subsys[] to get to blkio_cgroup, it would get NULL which
> is wrong. I was running into this race condition with ltp running on a
> upstream derived kernel and that lead to crash.
> 
> So ideally we should also fix cgroup generic code to wait for rcu
> grace period before setting pointer to NULL. Li Zefan is not very keen
> on introducing synchronize_wait() as he thinks it will slow
> down moun/remount/umount operations.
> 
> So for the time being atleast fix the kernel crash by taking a more
> direct route to blkio_cgroup.
> 
> One tester had reported a crash while running LTP on a derived kernel
> and with this fix crash is no more seen while the test has been
> running for over 6 days.
> 
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>

Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>