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Re: [PATCH] Fix incorrect double-checked locking in __nss_database_lookup. [BZ #20483]


On Mon, 2016-09-12 at 20:55 +0200, Florian Weimer wrote:
> On 08/18/2016 10:49 PM, Torvald Riegel wrote:
> >  DB_LOOKUP_FCT (service_user **ni, const char *fct_name, const char *fct2_name,
> >  	       void **fctp)
> >  {
> > -  if (DATABASE_NAME_SYMBOL == NULL
> > -      && __nss_database_lookup (DATABASE_NAME_STRING, ALTERNATE_NAME_STRING,
> > -				DEFAULT_CONFIG, &DATABASE_NAME_SYMBOL) < 0)
> > +  /* Relaxed MO is fine because this is just about whether we have to perform
> > +     the lookup; we will do another acquire-MO load next before assuming that
> > +     the lookup has happened.  */
> > +  if ((atomic_load_relaxed (&(DATABASE_NAME_SYMBOL)) == NULL)
> > +      && (__nss_database_lookup (DATABASE_NAME_STRING, ALTERNATE_NAME_STRING,
> > +				 DEFAULT_CONFIG, &DATABASE_NAME_SYMBOL) < 0))
> >      return -1;
> >
> > -  *ni = DATABASE_NAME_SYMBOL;
> > +  /* Acquire MO as required by __nss_database_lookup.  */
> > +  *ni = atomic_load_acquire (&(DATABASE_NAME_SYMBOL));
> >
> >    return __nss_lookup (ni, fct_name, fct2_name, fctp);
> >  }
> 
> I'm not sure if this is the double-checked locking pattern we want to 
> use for new code.  GCC currently cannot merge the two loads (and maybe 
> it never will do so because it is beneficial to treat the separate loads 
> as an optimization hint).

I don't think the (re)load matters performance-wise, given that
DATABASE_NAME_SYMBOL is initialized once, and so the access will most
likely hit in the cache.  What may matter were if we'd often returned -1
and would have an unnecessary acquire MO in this path, though OTOH
__nss_database_lookup will acquire the lock anyway.

Loading it into a temporary first, then doing the check, and then
issuing an acquire-MO fence would be another way to avoid the extra
load.

> I still think we would be better off if we centralize this particular 
> code in a single function with an interface similar to 
> __nss_database_lookup.  The manual inlining seems unnecessary.

Doing all of that in __nss_database_lookup might be cleaner, I agree.
But my focus for this patch was just a bug fix, not refactoring; I'd
like to leave it to others to decide how to clean-up this part of glibc.
I also wasn't quite sure whether __nss_database_lookup is part of the
ABI or not.


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