linux-kernel - Re: [PATCH 02/13] jump label v9: base patch

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Message-ID: <20100610154451.GC3923@redhat.com>
Date:	Thu, 10 Jun 2010 11:44:52 -0400
From:	Jason Baron <jbaron@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu,
	mathieu.desnoyers@...ymtl.ca, hpa@...or.com, tglx@...utronix.de,
	rostedt@...dmis.org, andi@...stfloor.org, roland@...hat.com,
	rth@...hat.com, mhiramat@...hat.com, avi@...hat.com,
	davem@...emloft.net, vgoyal@...hat.com, sam@...nborg.org
Subject: Re: [PATCH 02/13] jump label v9: base patch

On Thu, Jun 10, 2010 at 12:35:49AM +0200, Frederic Weisbecker wrote:
> On Wed, Jun 09, 2010 at 05:38:57PM -0400, Jason Baron wrote:
> > +static int build_jump_label_hashtable(struct jump_entry *start, struct jump_entry *stop)
> > +{
> > +	struct jump_entry *iter, *iter_begin;
> > +	struct jump_label_entry *entry;
> > +	int count;
> > +
> > +	sort_jump_label_entries(start, stop);
> > +	iter = start;
> > +	while (iter < stop) {
> > +		entry = get_jump_label_entry((char *)iter->name);
> > +		if (!entry) {
> > +			iter_begin = iter;
> > +			count = 0;
> > +			while ((iter < stop) &&
> > +				(strcmp((char *)iter->name,
> > +					(char *)iter_begin->name) == 0)) {
> > +				iter++;
> > +				count++;
> > +			}
> 
> 
> 
> 
> So, you can have multiple entries with the same name? How can that happen
> in fact?
> 
> 

this is the case where a single tracepoint such as kmalloc(), is used in
all over the kernel. So, there is one name or key value associated with
a kmalloc tracepoint. however, we have to patch the jump or nop into a
bunch of places in the kernel text.

> 
> 
> > +			entry = add_jump_label_entry((char *)iter_begin->name,
> > +							count, iter_begin);
> > +			if (IS_ERR(entry))
> > +				return PTR_ERR(entry);
> > +			continue;
> > +		}
> > +		WARN(1, KERN_ERR "build_jump_hashtable: unexpected entry!\n");
> 
> 
> 
> It seems you are going to endless loop in this fail case.
> 

agreed. I need to stick that 'WARN' into the else clause of "if
(!entry)" and return an error.

> 
> 
> > +	}
> > +	return 0;
> > +}
> > +
> > +/***
> > + * jump_label_update - update jump label text
> > + * @name - name of the jump label
> > + * @type - enum set to JUMP_LABEL_ENABLE or JUMP_LABEL_DISABLE
> > + *
> > + * Will enable/disable the jump for jump label @name, depending on the
> > + * value of @type.
> > + *
> > + */
> > +
> > +void jump_label_update(const char *name, enum jump_label_type type)
> > +{
> > +	struct jump_entry *iter;
> > +	struct jump_label_entry *entry;
> > +	struct hlist_node *module_node;
> > +	struct jump_label_module_entry *e_module;
> > +	int count;
> > +
> > +	mutex_lock(&jump_label_mutex);
> > +	entry = get_jump_label_entry(name);
> > +	if (entry) {
> > +		count = entry->nr_entries;
> > +		iter = entry->table;
> > +		while (count--) {
> > +			if (kernel_text_address(iter->code))
> > +				arch_jump_label_transform(iter, type);
> > +			iter++;
> > +		}
> 
> 
> 
> So, this is going to patch multiple times the same value on the
> same address in case you have multiple entries for the same name?
> 
> That look weird.

no. as mentioned before, there are multiple text addresses potentially
associated with a single jump label conditional variable. So we need to
patch all of them.

> 
> BTW, if you can't find the entry, you should perhaps propagate an error.
> 
> 
> 
> > +	}
> > +	mutex_unlock(&jump_label_mutex);
> > +}
> > +
> > +static int init_jump_label(void)
> 
> 
> 
> This can be __init.
>

ok.

thanks for the review.

-Jason 
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