[Fix,incorrect,double-checked,locking,related,to,_res_hconf.initialized.,[BZ,#20477]
Commit Message
_res_hconf.initialized was not suitable for use in a multi-threaded
environment due to the lack of atomics and memory barriers. Use of it
was also unnecessary because _res_hconf_init did the right thing by
using __libc_once. This patch fixes the glibc-internal uses by just
calling _res_hconf_init unconditionally, and switches to a release MO
atomic store for _res_hconf.initialized to fix the glibc side of the
synchronization problem (which will maintain backward compatibility, but
cannot fix the lack of acquire MO on any glibc-external loads).
[BZ #20477]
* resolv/res_hconf.c (do_init): Use atomic access.
* resolv/res_hconf.h: Add comments.
* nscd/aicache.c (addhstaiX): Call _res_hconf_init unconditionally.
* nss/getXXbyYY_r.c (REENTRANT_NAME): Likewise.
* sysdeps/posix/getaddrinfo.c (gaih_inet): Likewise.
Comments
On 08/17/2016 02:15 PM, Torvald Riegel wrote:
> _res_hconf.initialized was not suitable for use in a multi-threaded
> environment due to the lack of atomics and memory barriers. Use of it
> was also unnecessary because _res_hconf_init did the right thing by
> using __libc_once. This patch fixes the glibc-internal uses by just
> calling _res_hconf_init unconditionally, and switches to a release MO
> atomic store for _res_hconf.initialized to fix the glibc side of the
> synchronization problem (which will maintain backward compatibility, but
> cannot fix the lack of acquire MO on any glibc-external loads).
>
> [BZ #20477]
> * resolv/res_hconf.c (do_init): Use atomic access.
> * resolv/res_hconf.h: Add comments.
> * nscd/aicache.c (addhstaiX): Call _res_hconf_init unconditionally.
> * nss/getXXbyYY_r.c (REENTRANT_NAME): Likewise.
> * sysdeps/posix/getaddrinfo.c (gaih_inet): Likewise.
Looks good to me. Please commit.
Thanks,
Florian
On Aug 17 2016, Torvald Riegel <triegel@redhat.com> wrote:
> diff --git a/resolv/res_hconf.h b/resolv/res_hconf.h
> index b97734d..a3d23f3 100644
> --- a/resolv/res_hconf.h
> +++ b/resolv/res_hconf.h
> @@ -25,6 +25,15 @@
>
> struct hconf
> {
> + /* We keep the INITIALIZED member only for backwards compatibility. New
> + code should just call _res_hconf_init unconditionally. For this field
> + to be used safely, users must ensure that either (1) a call to
> + _res_hconf_init happens-before any load from INITIALIZED, or (2) an
^
> + assignment of zero to INITIALIZED happens-before any load from it, and
^
Those hyphens look wrong.
Andreas.
On 08/17/2016 04:51 PM, Andreas Schwab wrote:
> On Aug 17 2016, Torvald Riegel <triegel@redhat.com> wrote:
>
>> diff --git a/resolv/res_hconf.h b/resolv/res_hconf.h
>> index b97734d..a3d23f3 100644
>> --- a/resolv/res_hconf.h
>> +++ b/resolv/res_hconf.h
>> @@ -25,6 +25,15 @@
>>
>> struct hconf
>> {
>> + /* We keep the INITIALIZED member only for backwards compatibility. New
>> + code should just call _res_hconf_init unconditionally. For this field
>> + to be used safely, users must ensure that either (1) a call to
>> + _res_hconf_init happens-before any load from INITIALIZED, or (2) an
> ^
>> + assignment of zero to INITIALIZED happens-before any load from it, and
> ^
>
> Those hyphens look wrong.
“happens-before” is third-person singular of the verb “happen-before”.
It's a technical term widely used while discussing memory models.
Here's an example:
<http://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.4.5>
Florian
On Aug 17 2016, Florian Weimer <fweimer@redhat.com> wrote:
> On 08/17/2016 04:51 PM, Andreas Schwab wrote:
>> On Aug 17 2016, Torvald Riegel <triegel@redhat.com> wrote:
>>
>>> diff --git a/resolv/res_hconf.h b/resolv/res_hconf.h
>>> index b97734d..a3d23f3 100644
>>> --- a/resolv/res_hconf.h
>>> +++ b/resolv/res_hconf.h
>>> @@ -25,6 +25,15 @@
>>>
>>> struct hconf
>>> {
>>> + /* We keep the INITIALIZED member only for backwards compatibility. New
>>> + code should just call _res_hconf_init unconditionally. For this field
>>> + to be used safely, users must ensure that either (1) a call to
>>> + _res_hconf_init happens-before any load from INITIALIZED, or (2) an
>> ^
>>> + assignment of zero to INITIALIZED happens-before any load from it, and
>> ^
>>
>> Those hyphens look wrong.
>
> “happens-before” is third-person singular of the verb
> “happen-before”. It's a technical term widely used while discussing memory
> models.
But writing it without hyphen is standard English and doesn't require
special knowledge.
Andreas.
On 08/18/2016 09:05 AM, Andreas Schwab wrote:
> On Aug 17 2016, Florian Weimer <fweimer@redhat.com> wrote:
>
>> On 08/17/2016 04:51 PM, Andreas Schwab wrote:
>>> On Aug 17 2016, Torvald Riegel <triegel@redhat.com> wrote:
>>>
>>>> diff --git a/resolv/res_hconf.h b/resolv/res_hconf.h
>>>> index b97734d..a3d23f3 100644
>>>> --- a/resolv/res_hconf.h
>>>> +++ b/resolv/res_hconf.h
>>>> @@ -25,6 +25,15 @@
>>>>
>>>> struct hconf
>>>> {
>>>> + /* We keep the INITIALIZED member only for backwards compatibility. New
>>>> + code should just call _res_hconf_init unconditionally. For this field
>>>> + to be used safely, users must ensure that either (1) a call to
>>>> + _res_hconf_init happens-before any load from INITIALIZED, or (2) an
>>> ^
>>>> + assignment of zero to INITIALIZED happens-before any load from it, and
>>> ^
>>>
>>> Those hyphens look wrong.
>>
>> “happens-before” is third-person singular of the verb
>> “happen-before”. It's a technical term widely used while discussing memory
>> models.
>
> But writing it without hyphen is standard English and doesn't require
> special knowledge.
But making sense of this comment requires special knowledge, and writing
the verb as “happens-before” makes it clear that the memory relation is
meant, and not some foggy concept of ordering. It's like
MAY/SHOULD/RECOMMEND in IETF RFCs in this regard.
Florian
On Thu, 2016-08-18 at 10:05 +0200, Florian Weimer wrote:
> On 08/18/2016 09:05 AM, Andreas Schwab wrote:
> > On Aug 17 2016, Florian Weimer <fweimer@redhat.com> wrote:
> >
> >> On 08/17/2016 04:51 PM, Andreas Schwab wrote:
> >>> On Aug 17 2016, Torvald Riegel <triegel@redhat.com> wrote:
> >>>
> >>>> diff --git a/resolv/res_hconf.h b/resolv/res_hconf.h
> >>>> index b97734d..a3d23f3 100644
> >>>> --- a/resolv/res_hconf.h
> >>>> +++ b/resolv/res_hconf.h
> >>>> @@ -25,6 +25,15 @@
> >>>>
> >>>> struct hconf
> >>>> {
> >>>> + /* We keep the INITIALIZED member only for backwards compatibility. New
> >>>> + code should just call _res_hconf_init unconditionally. For this field
> >>>> + to be used safely, users must ensure that either (1) a call to
> >>>> + _res_hconf_init happens-before any load from INITIALIZED, or (2) an
> >>> ^
> >>>> + assignment of zero to INITIALIZED happens-before any load from it, and
> >>> ^
> >>>
> >>> Those hyphens look wrong.
> >>
> >> “happens-before” is third-person singular of the verb
> >> “happen-before”. It's a technical term widely used while discussing memory
> >> models.
> >
> > But writing it without hyphen is standard English and doesn't require
> > special knowledge.
>
> But making sense of this comment requires special knowledge, and writing
> the verb as “happens-before” makes it clear that the memory relation is
> meant, and not some foggy concept of ordering. It's like
> MAY/SHOULD/RECOMMEND in IETF RFCs in this regard.
Yes. As Florian says, this is meant to refer to the happens-before
relation in the C11 memory model, and not some other notion of ordering.
The formalization of the C11 memory model uses "happens-before", so it
seemed like a good choice.
Nonetheless, I'm open to other ways of referring to it, as long as it's
precise and clearly refers to the memory model; in the end, the glibc
community needs to be happy with how we talk about concurrency. If
anyone has suggestions, let me know.
commit 44d3ca295ade8cb11b1473a7009705f6ec99d9bf
Author: Torvald Riegel <triegel@redhat.com>
Date: Wed Aug 17 13:56:11 2016 +0200
Fix incorrect double-checked locking related to _res_hconf.initialized.
_res_hconf.initialized was not suitable for use in a multi-threaded
environment due to the lack of atomics and memory barriers. Use of it was
also unnecessary because _res_hconf_init did the right thing by using
__libc_once. This patch fixes the glibc-internal uses by just calling
_res_hconf_init unconditionally, and switches to a release MO atomic store
for _res_hconf.initialized to fix the glibc side of the synchronization
problem (which will maintain backward compatibility, but cannot fix the
lack of acquire MO on any glibc-external loads).
[BZ #20477]
* resolv/res_hconf.c (do_init): Use atomic access.
* resolv/res_hconf.h: Add comments.
* nscd/aicache.c (addhstaiX): Call _res_hconf_init unconditionally.
* nss/getXXbyYY_r.c (REENTRANT_NAME): Likewise.
* sysdeps/posix/getaddrinfo.c (gaih_inet): Likewise.
@@ -101,8 +101,7 @@ addhstaiX (struct database_dyn *db, int fd, request_header *req,
nip = hosts_database;
/* Initialize configurations. */
- if (__glibc_unlikely (!_res_hconf.initialized))
- _res_hconf_init ();
+ _res_hconf_init ();
if (__res_maybe_init (&_res, 0) == -1)
no_more = 1;
@@ -274,8 +274,7 @@ INTERNAL (REENTRANT_NAME) (ADD_PARAMS, LOOKUP_TYPE *resbuf, char *buffer,
}
#endif /* need _res */
#ifdef NEED__RES_HCONF
- if (!_res_hconf.initialized)
- _res_hconf_init ();
+ _res_hconf_init ();
#endif /* need _res_hconf */
void *tmp_ptr = fct.l;
@@ -348,7 +348,8 @@ do_init (void)
arg_trimdomain_list (ENV_TRIM_OVERR, 1, envval);
}
- _res_hconf.initialized = 1;
+ /* See comments on the declaration of _res_hconf. */
+ atomic_store_release (&_res_hconf.initialized, 1);
}
@@ -25,6 +25,15 @@
struct hconf
{
+ /* We keep the INITIALIZED member only for backwards compatibility. New
+ code should just call _res_hconf_init unconditionally. For this field
+ to be used safely, users must ensure that either (1) a call to
+ _res_hconf_init happens-before any load from INITIALIZED, or (2) an
+ assignment of zero to INITIALIZED happens-before any load from it, and
+ these loads use acquire MO if the intent is to skip calling
+ _res_hconf_init if the load returns a nonzero value. Such acquire MO
+ loads will then synchronize with the release MO store to INITIALIZED
+ in do_init in res_hconf.c; see pthread_once for more detail. */
int initialized;
int unused1;
int unused2[4];
@@ -816,8 +816,7 @@ gaih_inet (const char *name, const struct gaih_service *service,
nip = __nss_hosts_database;
/* Initialize configurations. */
- if (__glibc_unlikely (!_res_hconf.initialized))
- _res_hconf_init ();
+ _res_hconf_init ();
if (__res_maybe_init (&_res, 0) == -1)
no_more = 1;