Re: [RFC][PATCH -mm 1/5] PM: Make freeze_processes SMP-safe

Rafael J. Wysocki
2006-11-26T10:02:16Z

Hi,

On Sunday, 26 November 2006 08:47, Pavel Machek wrote:
> Hi!
>
> > Currently, the PF_FREEZE process flag is used to indicate that the process
> > should enter the refrigerator as soon as possible. Unfortunately it is set by
> > the freezer while the process may be changing its flags for another reason
> > and this may lead to a race between the freezer and the process itself.
> >
> > This problem may be solved by introducing an additional member, called (for
> > example) 'freezing', into task_struct which will only be used to indicate that
> > the process should enter the refrigerator. Then, if the 'freezing' member of
> > task_struct is reset by the process itself only after it has entered the
> > refrigerator, the modifications of it will be guaranteed to occur at different
> > times, because the freezer can only set it before the process enters the
> > refrigerator. Thus the code will be SMP-safe even though no explicit locking
> > is used.
>
> I do not think we can go without locking here.

Why exactly?

> > @@ -31,7 +30,7 @@ static inline void freeze(struct task_st
> > */
> > static inline void do_not_freeze(struct task_struct *p)
> > {
> > - p->flags &= ~PF_FREEZE;
> > + p->freezing = 0;
> > }
> >
> > /*
> > @@ -52,7 +51,8 @@ static inline int thaw_process(struct ta
> > */
> > static inline void frozen_process(struct task_struct *p)
> > {
> > - p->flags = (p->flags & ~PF_FREEZE) | PF_FROZEN;
> > + p->flags |= PF_FROZEN;
> > + p->freezing = 0;
> > }
>
> Is mb() needed between |= and freezing = 0?

Hm, I'm not sure. Are there architectures on which memory writes can be
reordered?

> > extern void refrigerator(void);
> > Index: linux-2.6.19-rc6-mm1/include/linux/sched.h
> > ===================================================================
> > --- linux-2.6.19-rc6-mm1.orig/include/linux/sched.h
> > +++ linux-2.6.19-rc6-mm1/include/linux/sched.h
> > @@ -1065,6 +1065,9 @@ struct task_struct {
> > #ifdef CONFIG_TASK_DELAY_ACCT
> > struct task_delay_info *delays;
> > #endif
> > +#ifdef CONFIG_PM
> > + int freezing; /* if set, we should be freezing for suspend */
> > +#endif
> > #ifdef CONFIG_FAULT_INJECTION
> > int make_it_fail;
> > #endif
>
> It is int, imagine machine that can't do 32-bit atomic access (only
> does 64 bits). On such beast (alpha? something stranger?) this will
> clobber make_it_fail field, sometimes.
>
> OTOH on i386 normal instructions can be used. But that's okay, we
> should just use atomic_t here. Should be as fast on i386/x86-64, and
> still safe.

Okay, I'll use atomic_t.

Greetings,
Rafael

--
You never change things by fighting the existing reality.
R. Buckminster Fuller