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  • Vivek Goyal's avatar
    blk-throttle: Use task_subsys_state() to determine a task's blkio_cgroup · 70087dc3
    Vivek Goyal authored
    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: default avatarVivek Goyal <>
    Reviewed-by: default avatarLi Zefan <>
    Signed-off-by: default avatarJens Axboe <>