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 <[email protected] > wrote: > Sorry, didn't reply to the list: > > On Fri, Apr 27, 2018 at 1:48 AM, Stephen J. Butler < > [email protected]> 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 < >> [email protected]> 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 < >>> [email protected]> 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 >>>> [email protected] >>>> https://www.varnish-cache.org/lists/mailman/listinfo/varnish-dev >>>> >>> >>> >> >
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