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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 Wed, 26 Jan 2011 17:08:24 -0800
Andrew Morton <akpm@linux-foundation.org> wrote:

> 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.
>

seq_counter can't be used for res_counter because I don't want to update
seq_counter at changing ->usae of res_counter. So, I'd like to just add
spinlock for this time.

I wonder making memcg's counter to use 32bit and record usage in the number
of pages may be a simple way...

Thanks,
-Kame



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