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    SubjectRe: [PATCH] memcg: restore ss->id_lock to spinlock, using RCU for next
    Hugh Dickins wrote:
    > Commit c1e2ee2dc436 "memcg: replace ss->id_lock with a rwlock" has
    > now been seen to cause the unfair behavior we should have expected
    > from converting a spinlock to an rwlock: softlockup in cgroup_mkdir(),
    > whose get_new_cssid() is waiting for the wlock, while there are 19
    > tasks using the rlock in css_get_next() to get on with their memcg
    > workload (in an artificial test, admittedly). Yet lib/idr.c was
    > made suitable for RCU way back.
    >
    > 1. Revert that commit, restoring ss->id_lock to a spinlock.
    >
    > 2. Make one small adjustment to idr_get_next(): take the height from
    > the top layer (stable under RCU) instead of from the root (unprotected
    > by RCU), as idr_find() does.
    >
    > 3. Remove lock and unlock around css_get_next()'s call to idr_get_next():
    > memcg iterators (only users of css_get_next) already did rcu_read_lock(),
    > and comment demands that, but add a WARN_ON_ONCE to make sure of it.
    >
    > Signed-off-by: Hugh Dickins <hughd@google.com>

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

    > ---
    >
    > include/linux/cgroup.h | 2 +-
    > kernel/cgroup.c | 19 +++++++++----------
    > lib/idr.c | 4 ++--
    > 3 files changed, 12 insertions(+), 13 deletions(-)


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