linux-kernel - Re: About 'hash' event trigger patchset

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Date:	Fri, 04 Apr 2014 16:36:50 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Tom Zanussi <tom.zanussi@...ux.intel.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: About 'hash' event trigger patchset

Hi Tom,

On Wed, 02 Apr 2014 09:51:54 -0500, Tom Zanussi wrote:
> Hi Namhyung,
>
> On Wed, 2014-04-02 at 08:31 +0000, Namhyung Kim wrote:
>> One thing I noticed in the main logic is that it seems there's no
>> limit checking when adding/creating new entry.  In
>> hash_trigger_entry_create(), there's a check against max_entries but
>> if it goes beyond the max, it'd just access a NULL pointer AFAICS.  Am
>> I missing something?  Also I don't know what the difference between
>> ->n_entries and ->total_entries (in hash_data).
>> 
>> I guess you wanted to set ->drops in that case, but I cannot find
>
> Yes, the code is missing a very important snippet, which I realized
> after hitting the problem.  My current code has this:
>
>         if (hash_data->drops)
>                 return NULL;

I think this part can be omitted since it's already checked earlier.
But it's a minor issue.


>         else if (hash_data->n_entries == hash_data->max_entries) {
>                 hash_data->drops = 1;
>                 return NULL;
>         }
>
> n_entries is the current number of entries used up, and max_entries is
> the total number of available entries (a cached value to avoid
> calculating it every time).

But there's "total_entries" - increased in hash_trigger_entry_insert() -
too and I think it's just same as n_entries.

>
>> where it gets set.  And I'm not sure it's good to check ->drop first,
>> since entry can find an existing entry and merged to it even if it
>> reached the max already.
>> 
>
> The assumption is that if you have any drops at all, you probably want
> to redo the test with a bigger table, but regardless the data reflects
> the situation up to the point the drops started happening.  Letting
> events that already have a entry merge while rejecting those that don't
> would invalidate the data you already have.

Okay, I won't insist on it.

Thanks,
Namhyung
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