diff mbox

[06/24] dm cache policy mq: return NULL if mq->free list is empty in alloc_entry

Message ID 1382639437-27007-7-git-send-email-snitzer@redhat.com (mailing list archive)
State Superseded, archived
Delegated to: Mike Snitzer
Headers show

Commit Message

Mike Snitzer Oct. 24, 2013, 6:30 p.m. UTC 
From: Heinz Mauelshagen <heinzm@redhat.com>

Addresses callers' (insert_in*cache()) requirement that alloc_entry()
return NULL when an entry isn't able to be allocated.

Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 drivers/md/dm-cache-policy-mq.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

Comments

Alasdair G Kergon Oct. 25, 2013, 4:37 p.m. UTC | #1 
On Thu, Oct 24, 2013 at 02:30:19PM -0400, Mike Snitzer wrote:
> From: Heinz Mauelshagen <heinzm@redhat.com>
> Addresses callers' (insert_in*cache()) requirement that alloc_entry()
> return NULL when an entry isn't able to be allocated.
 
What is the code path that leads to the requirement for this patch?

> +++ b/drivers/md/dm-cache-policy-mq.c
>  static struct entry *alloc_entry(struct mq_policy *mq)

> +	struct entry *e = NULL;
>  
>  	if (mq->nr_entries_allocated >= mq->nr_entries) {
>  		BUG_ON(!list_empty(&mq->free));
>  		return NULL;
>  	}
>  
> -	e = list_entry(list_pop(&mq->free), struct entry, list);
> -	INIT_LIST_HEAD(&e->list);
> -	INIT_HLIST_NODE(&e->hlist);
> +	if (!list_empty(&mq->free)) {
> +		e = list_entry(list_pop(&mq->free), struct entry, list);
> +		INIT_LIST_HEAD(&e->list);
> +		INIT_HLIST_NODE(&e->hlist);
> +		mq->nr_entries_allocated++;
> +	}

In other words, under what circumstances is mq->nr_entries_allocated
less then mq->nr_entries, yet the mq->free list is empty?

Is it better to apply a patch like this, or rather to fix whatever situation
leads to those circumstances?  Has the bug/race always been there or is it a
regression?

Alasdair

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Mike Snitzer Oct. 25, 2013, 8:44 p.m. UTC | #2 
On Fri, Oct 25 2013 at 12:37pm -0400,
Alasdair G Kergon <agk@redhat.com> wrote:

> On Thu, Oct 24, 2013 at 02:30:19PM -0400, Mike Snitzer wrote:
> > From: Heinz Mauelshagen <heinzm@redhat.com>
> > Addresses callers' (insert_in*cache()) requirement that alloc_entry()
> > return NULL when an entry isn't able to be allocated.
>  
> What is the code path that leads to the requirement for this patch?
> 
> > +++ b/drivers/md/dm-cache-policy-mq.c
> >  static struct entry *alloc_entry(struct mq_policy *mq)
> 
> > +	struct entry *e = NULL;
> >  
> >  	if (mq->nr_entries_allocated >= mq->nr_entries) {
> >  		BUG_ON(!list_empty(&mq->free));
> >  		return NULL;
> >  	}
> >  
> > -	e = list_entry(list_pop(&mq->free), struct entry, list);
> > -	INIT_LIST_HEAD(&e->list);
> > -	INIT_HLIST_NODE(&e->hlist);
> > +	if (!list_empty(&mq->free)) {
> > +		e = list_entry(list_pop(&mq->free), struct entry, list);
> > +		INIT_LIST_HEAD(&e->list);
> > +		INIT_HLIST_NODE(&e->hlist);
> > +		mq->nr_entries_allocated++;
> > +	}
> 
> In other words, under what circumstances is mq->nr_entries_allocated
> less then mq->nr_entries, yet the mq->free list is empty?
> 
> Is it better to apply a patch like this, or rather to fix whatever situation
> leads to those circumstances?  Has the bug/race always been there or is it a
> regression?

Fair questions, Heinz should explain further.

IIRC this change was needed as a prereq for the conversion of his
out-of-tree "background" policy to a shim (layered ontop of mq).

So will drop for now, can revisit if/when the need is clearer (e.g. when
background policy goes upstream).  TBD if we need it in the near-term,
but will table this for now.  Dropping patch until more context from
Heinz is provided.

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Heinz Mauelshagen Oct. 25, 2013, 10:36 p.m. UTC | #3 
On 10/25/2013 10:44 PM, Mike Snitzer wrote:
> On Fri, Oct 25 2013 at 12:37pm -0400,
> Alasdair G Kergon <agk@redhat.com> wrote:
>
>> On Thu, Oct 24, 2013 at 02:30:19PM -0400, Mike Snitzer wrote:
>>> From: Heinz Mauelshagen <heinzm@redhat.com>
>>> Addresses callers' (insert_in*cache()) requirement that alloc_entry()
>>> return NULL when an entry isn't able to be allocated.
>>   
>> What is the code path that leads to the requirement for this patch?
>>
>>> +++ b/drivers/md/dm-cache-policy-mq.c
>>>   static struct entry *alloc_entry(struct mq_policy *mq)
>>> +	struct entry *e = NULL;
>>>   
>>>   	if (mq->nr_entries_allocated >= mq->nr_entries) {
>>>   		BUG_ON(!list_empty(&mq->free));
>>>   		return NULL;
>>>   	}
>>>   
>>> -	e = list_entry(list_pop(&mq->free), struct entry, list);
>>> -	INIT_LIST_HEAD(&e->list);
>>> -	INIT_HLIST_NODE(&e->hlist);
>>> +	if (!list_empty(&mq->free)) {
>>> +		e = list_entry(list_pop(&mq->free), struct entry, list);
>>> +		INIT_LIST_HEAD(&e->list);
>>> +		INIT_HLIST_NODE(&e->hlist);
>>> +		mq->nr_entries_allocated++;
>>> +	}
>> In other words, under what circumstances is mq->nr_entries_allocated
>> less then mq->nr_entries, yet the mq->free list is empty?
>>
>> Is it better to apply a patch like this, or rather to fix whatever situation
>> leads to those circumstances?  Has the bug/race always been there or is it a
>> regression?
> Fair questions, Heinz should explain further.
>
> IIRC this change was needed as a prereq for the conversion of his
> out-of-tree "background" policy to a shim (layered ontop of mq).

Has nothing to do with that.

It is a fix for existing callers presuming that alloc_entry() could 
return NULL but the
callee did not handle this properly.


>
> So will drop for now, can revisit if/when the need is clearer (e.g. when
> background policy goes upstream).  TBD if we need it in the near-term,
> but will table this for now.  Dropping patch until more context from
> Heinz is provided.

Leave it in for the aforementioned reason.

Heinz

>
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diff mbox

Patch

diff --git a/drivers/md/dm-cache-policy-mq.c b/drivers/md/dm-cache-policy-mq.c
index 108380d..35e6798 100644
--- a/drivers/md/dm-cache-policy-mq.c
+++ b/drivers/md/dm-cache-policy-mq.c
@@ -399,18 +399,20 @@  static void hash_remove(struct entry *e)
  */
 static struct entry *alloc_entry(struct mq_policy *mq)
 {
-	struct entry *e;
+	struct entry *e = NULL;
 
 	if (mq->nr_entries_allocated >= mq->nr_entries) {
 		BUG_ON(!list_empty(&mq->free));
 		return NULL;
 	}
 
-	e = list_entry(list_pop(&mq->free), struct entry, list);
-	INIT_LIST_HEAD(&e->list);
-	INIT_HLIST_NODE(&e->hlist);
+	if (!list_empty(&mq->free)) {
+		e = list_entry(list_pop(&mq->free), struct entry, list);
+		INIT_LIST_HEAD(&e->list);
+		INIT_HLIST_NODE(&e->hlist);
+		mq->nr_entries_allocated++;
+	}
 
-	mq->nr_entries_allocated++;
 	return e;
 }