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    SubjectRe: [BUGFIX] memcg: fix res_counter_read_u64 lock aware (Was Re: [PATCH] oom: handle overflow in mem_cgroup_out_of_memory()
    On Thu, 27 Jan 2011 09:53:42 +0900
    KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

    > res_counter_read_u64 reads u64 value without lock. It's dangerous
    > in 32bit environment. This patch adds lock.
    >
    > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    > ---
    > include/linux/res_counter.h | 13 ++++++++++++-
    > kernel/res_counter.c | 2 +-
    > 2 files changed, 13 insertions(+), 2 deletions(-)
    >
    > Index: mmotm-0125/include/linux/res_counter.h
    > ===================================================================
    > --- mmotm-0125.orig/include/linux/res_counter.h
    > +++ mmotm-0125/include/linux/res_counter.h
    > @@ -68,7 +68,18 @@ struct res_counter {
    > * @pos: and the offset.
    > */
    >
    > -u64 res_counter_read_u64(struct res_counter *counter, int member);
    > +u64 res_counter_read_u64_locked(struct res_counter *counter, int member);
    > +
    > +static inline u64 res_counter_read_u64(struct res_counter *counter, int member)
    > +{
    > + unsigned long flags;
    > + u64 ret;
    > +
    > + spin_lock_irqsave(&counter->lock, flags);
    > + ret = res_counter_read_u64_locked(counter, member);
    > + spin_unlock_irqrestore(&counter->lock, flags);
    > + return ret;
    > +}
    >
    > ssize_t res_counter_read(struct res_counter *counter, int member,
    > const char __user *buf, size_t nbytes, loff_t *pos,
    > Index: mmotm-0125/kernel/res_counter.c
    > ===================================================================
    > --- mmotm-0125.orig/kernel/res_counter.c
    > +++ mmotm-0125/kernel/res_counter.c
    > @@ -126,7 +126,7 @@ ssize_t res_counter_read(struct res_coun
    > pos, buf, s - buf);
    > }
    >
    > -u64 res_counter_read_u64(struct res_counter *counter, int member)
    > +u64 res_counter_read_u64_locked(struct res_counter *counter, int member)
    > {
    > return *res_counter_member(counter, member);
    > }

    We don't need the lock on 64-bit platforms!

    And there's zero benefit to inlining the spin_lock/unlock(), given that
    the function will always be making a function call anyway.

    See i_size_read() for inspiration.


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