PRIV_CALL, VRT_re_init, and mutexes

[Stephen J. Butler]

(sorry, never mind about the -- and ++; they're all inside the lock)

On Fri, Apr 27, 2018 at 2:05 AM, Stephen J. Butler <stephen.butler at gmail.com
> wrote:

> In the varnish source code for the 6.0 release, I found one use of
> PRIV_CALL in vmod_std, vmod_fileread(). This function does have locks, but
> they're all to protect modifications of the global frlist variable. It does
> seem like the priv parameter is used as a call site value but no care is
> taken to make sure access is exclusive.
>
> In that case it also bothers me that there's an assumption that -- and ++
> are atomic operations on all platforms and compilers... some quick Googling
> suggests that's not the case.
>
>
> On Fri, Apr 27, 2018 at 1:49 AM, Stephen J. Butler <
> stephen.butler at gmail.com> wrote:
>
>> Sorry, didn't reply to the list:
>>
>> On Fri, Apr 27, 2018 at 1:48 AM, Stephen J. Butler <
>> stephen.butler at gmail.com> wrote:
>>
>>> Hmm. If PRIV_CALL is private to the call site, then I think
>>> in vmod_bodyaccess.c, vmod_rematch_req_body()there's a problem. In that
>>> case it calls VRE_compile()to initiate a regex without a lock on a
>>> PRIV_CALL structure.
>>>
>>> On Fri, Apr 27, 2018 at 1:37 AM, Guillaume Quintard <
>>> guillaume at varnish-software.com> wrote:
>>>
>>>> Hi, and welcome!
>>>>
>>>> In this case, priv comes from PRIV_CALL, meaning it's going to be the
>>>> same for all the request/vcl that call the function from the same spot.
>>>> This is done to cache the regex, so you only compile for the first call.
>>>>
>>>> However, I'm thinking that you may have the same priv with a different
>>>> string, and then everything goes to hell. Could anyone familiar with
>>>> PRIV_CALL confirm?
>>>>
>>>> Cheers,
>>>>
>>>> --
>>>> Guillaume Quintard
>>>>
>>>> On Fri, Apr 27, 2018 at 8:05 AM, Stephen J. Butler <
>>>> stephen.butler at gmail.com> wrote:
>>>>
>>>>> I'm a new developer/user for Varnish, and was looking to extend a
>>>>> module with some regex support (vmod_cookie if you must know). I don't know
>>>>> anything about vmod development or Varnish threading, so to get an idea of
>>>>> how to do this I looked at the vmod_header module.
>>>>>
>>>>> But I'm confused about why they thought they needed a mutex to
>>>>> protected a call to VRT_re_init. Is this just a useless, historic thing
>>>>> that got left in for some reason?
>>>>>
>>>>> https://github.com/varnish/varnish-modules/blob/master/src/v
>>>>> mod_header.c
>>>>>
>>>>> If you look at vmod_remove it has a 2nd parameter that's in the vcc as
>>>>> PRIV_CALL. This parameter is initialized with the string value from the
>>>>> last parameter (a regex pattern). It does this by calling
>>>>> header_init_re(). And if you look at that function you see:
>>>>>
>>>>> /*
>>>>> * Initialize the regex *s on priv, if it hasn't already been done.
>>>>> * XXX: We have to recheck the condition after grabbing the lock to
>>>>> avoid a
>>>>> * XXX: race condition.
>>>>> */
>>>>> static void
>>>>> header_init_re(struct vmod_priv *priv, const char *s)
>>>>> {
>>>>> if (priv->priv == NULL) {
>>>>> assert(pthread_mutex_lock(&header_mutex) == 0);
>>>>> if (priv->priv == NULL) {
>>>>> VRT_re_init(&priv->priv, s);
>>>>> priv->free = VRT_re_fini;
>>>>> }
>>>>> pthread_mutex_unlock(&header_mutex);
>>>>> }
>>>>> }
>>>>> My reading of the docs makes me think that this "priv" only lives for
>>>>> each single call of the function. There should be no reason to protect it
>>>>> with a global mutex. Do the internals of VRT_re_init() require this? I
>>>>> can't imagine how, because if it did then this method of protecting it is
>>>>> broken anyway.
>>>>>
>>>>> I suspect this is leftover from some rewrite or updating of the module
>>>>> but wanted to check.
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> varnish-dev mailing list
>>>>> varnish-dev at varnish-cache.org
>>>>> https://www.varnish-cache.org/lists/mailman/listinfo/varnish-dev
>>>>>
>>>>
>>>>
>>>
>>
>
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