Re: [PATCH] memcg: restore ss->id_lock to spinlock, using RCU fornext

[Li Zefan]

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@xxxxxxxxxx>

Acked-by: Li Zefan <lizf@xxxxxxxxxxxxxx>

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