Re: [PATCH 1/2] module: use rcu to protect module list read

[Cong Wang]

On 03/13/2012 08:32 AM, Rusty Russell wrote:
> On Sat, 10 Mar 2012 22:20:02 +0800, Cong Wang<xiyou.wangcong@xxxxxxxxx>  wrote:
> > Now the read of module list is protected by preempt disable + *_rcu
> > list operations, this is odd, as RCU read lock should be able to
> > protect it directly. This patch makes the read of module list
> > protected by RCU read lock and the write still protected by
> > module_mutex.
>
> OK, please split these patches further.  Locking is subtle, so it's
> great to be able to bisect more finely if we catch a problem.
>
> eg.  First replace all the preempt_disable()/enable with
> rcu_read_lock()/unlock.  Then replace lock in set_all_modules_text.
> And so on...

Fair enough, will do!

> > @@ -1810,11 +1810,11 @@ void *__symbol_get(const char *symbol)
> > 	struct module *owner;
> > 	const struct kernel_symbol *sym;
> >
> > -	preempt_disable();
> > +	rcu_read_lock();
> > 	sym = find_symbol(symbol,&owner, NULL, true, true);
> > +	rcu_read_unlock();
> > 	if (sym&&  strong_try_module_get(owner))
> > 		sym = NULL;
> > -	preempt_enable();
> >
> > 	return sym ? (void *)sym->value : NULL;
> >   }
>
> This is wrong: the symbol can vanish between find_symbol() and
> strong_try_module_get().  We need protection around the whole thing.

This is my mistake. Sorry. :(

> > @@ -3302,7 +3309,7 @@ static char *module_flags(struct module *mod, char *buf)
> >   /* Called by the /proc file system to return a list of modules. */
> >   static void *m_start(struct seq_file *m, loff_t *pos)
> >   {
> > -	mutex_lock(&module_mutex);
> > +	rcu_read_lock();
> >   	return seq_list_start(&modules, *pos);
> >   }
> >
> > @@ -3313,7 +3320,7 @@ static void *m_next(struct seq_file *m, void *p, loff_t *pos)
> >
> >   static void m_stop(struct seq_file *m, void *p)
> >   {
> > -	mutex_unlock(&module_mutex);
> > +	rcu_read_unlock();
> >   }
> >
> >   static int m_show(struct seq_file *m, void *p)
>
> Interesting.  I assume that these functions needed to sleep.  But it
> looks like I was wrong.

I didn't touch this part in V2, because seq_list_start() doesn't use 
_rcu operations on the list. Maybe we need a seq_list_start_rcu()?

Thanks!
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