* [PATCH] libstdc++: Fix aliasing violation in std::shared_ptr
@ 2022-01-21 13:49 Jonathan Wakely
2022-01-23 22:51 ` Jonathan Wakely
0 siblings, 1 reply; 3+ messages in thread
From: Jonathan Wakely @ 2022-01-21 13:49 UTC (permalink / raw)
To: libstdc++, gcc-patches
Tested powerpc64le-linux. Does anybody see a problem with this change?
The non-atomic store that sets both reference counts to zero uses a
type-punned pointer, which has undefined behaviour. We could use memset
to write 8 bytes, but we don't actually need it to be a single store
anyway. No other thread can observe the values, that's why it's safe to
use non-atomic stores in the first place. So we can just set each count
to zero.
With -fstore-merging (which is enabled by default at -O2) GCC produces
the same code for this as for memset or the type punned store. Clang
does that store merging even at -O1.
libstdc++-v3/ChangeLog:
* include/bits/shared_ptr_base.h (_Sp_counted_base<>::_M_release):
Set members to zero without type punning.
---
libstdc++-v3/include/bits/shared_ptr_base.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/libstdc++-v3/include/bits/shared_ptr_base.h b/libstdc++-v3/include/bits/shared_ptr_base.h
index 5b8f84b65be..b2f955b41f7 100644
--- a/libstdc++-v3/include/bits/shared_ptr_base.h
+++ b/libstdc++-v3/include/bits/shared_ptr_base.h
@@ -340,7 +340,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
// we are releasing the last strong reference. No other
// threads can observe the effects of this _M_release()
// call (e.g. calling use_count()) without a data race.
- *(long long*)(&_M_use_count) = 0;
+ _M_weak_count = _M_use_count = 0;
_GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count);
_GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);
_M_dispose();
--
2.31.1
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] libstdc++: Fix aliasing violation in std::shared_ptr
2022-01-21 13:49 [PATCH] libstdc++: Fix aliasing violation in std::shared_ptr Jonathan Wakely
@ 2022-01-23 22:51 ` Jonathan Wakely
2022-01-24 4:46 ` Maged Michael
0 siblings, 1 reply; 3+ messages in thread
From: Jonathan Wakely @ 2022-01-23 22:51 UTC (permalink / raw)
To: Jonathan Wakely; +Cc: libstdc++, gcc Patches, Maged Michael
I thought I'd CC'd Maged on this patch, but apparently not. I've
pushed it to trunk now.
On Fri, 21 Jan 2022 at 13:50, Jonathan Wakely wrote:
>
> Tested powerpc64le-linux. Does anybody see a problem with this change?
>
>
> The non-atomic store that sets both reference counts to zero uses a
> type-punned pointer, which has undefined behaviour. We could use memset
> to write 8 bytes, but we don't actually need it to be a single store
> anyway. No other thread can observe the values, that's why it's safe to
> use non-atomic stores in the first place. So we can just set each count
> to zero.
>
> With -fstore-merging (which is enabled by default at -O2) GCC produces
> the same code for this as for memset or the type punned store. Clang
> does that store merging even at -O1.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/shared_ptr_base.h (_Sp_counted_base<>::_M_release):
> Set members to zero without type punning.
> ---
> libstdc++-v3/include/bits/shared_ptr_base.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/libstdc++-v3/include/bits/shared_ptr_base.h b/libstdc++-v3/include/bits/shared_ptr_base.h
> index 5b8f84b65be..b2f955b41f7 100644
> --- a/libstdc++-v3/include/bits/shared_ptr_base.h
> +++ b/libstdc++-v3/include/bits/shared_ptr_base.h
> @@ -340,7 +340,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> // we are releasing the last strong reference. No other
> // threads can observe the effects of this _M_release()
> // call (e.g. calling use_count()) without a data race.
> - *(long long*)(&_M_use_count) = 0;
> + _M_weak_count = _M_use_count = 0;
> _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count);
> _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);
> _M_dispose();
> --
> 2.31.1
>
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] libstdc++: Fix aliasing violation in std::shared_ptr
2022-01-23 22:51 ` Jonathan Wakely
@ 2022-01-24 4:46 ` Maged Michael
0 siblings, 0 replies; 3+ messages in thread
From: Maged Michael @ 2022-01-24 4:46 UTC (permalink / raw)
To: Jonathan Wakely; +Cc: libstdc++, gcc Patches
Thanks Jonathan!
On Sun, Jan 23, 2022 at 5:51 PM Jonathan Wakely <jwakely@redhat.com> wrote:
> I thought I'd CC'd Maged on this patch, but apparently not. I've
> pushed it to trunk now.
>
> On Fri, 21 Jan 2022 at 13:50, Jonathan Wakely wrote:
> >
> > Tested powerpc64le-linux. Does anybody see a problem with this change?
> >
> >
> > The non-atomic store that sets both reference counts to zero uses a
> > type-punned pointer, which has undefined behaviour. We could use memset
> > to write 8 bytes, but we don't actually need it to be a single store
> > anyway. No other thread can observe the values, that's why it's safe to
> > use non-atomic stores in the first place. So we can just set each count
> > to zero.
> >
> > With -fstore-merging (which is enabled by default at -O2) GCC produces
> > the same code for this as for memset or the type punned store. Clang
> > does that store merging even at -O1.
> >
> > libstdc++-v3/ChangeLog:
> >
> > * include/bits/shared_ptr_base.h
> (_Sp_counted_base<>::_M_release):
> > Set members to zero without type punning.
> > ---
> > libstdc++-v3/include/bits/shared_ptr_base.h | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/libstdc++-v3/include/bits/shared_ptr_base.h
> b/libstdc++-v3/include/bits/shared_ptr_base.h
> > index 5b8f84b65be..b2f955b41f7 100644
> > --- a/libstdc++-v3/include/bits/shared_ptr_base.h
> > +++ b/libstdc++-v3/include/bits/shared_ptr_base.h
> > @@ -340,7 +340,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > // we are releasing the last strong reference. No other
> > // threads can observe the effects of this _M_release()
> > // call (e.g. calling use_count()) without a data race.
> > - *(long long*)(&_M_use_count) = 0;
> > + _M_weak_count = _M_use_count = 0;
> > _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count);
> > _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);
> > _M_dispose();
> > --
> > 2.31.1
> >
>
>
^ permalink raw reply [flat|nested] 3+ messages in thread
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2022-01-21 13:49 [PATCH] libstdc++: Fix aliasing violation in std::shared_ptr Jonathan Wakely
2022-01-23 22:51 ` Jonathan Wakely
2022-01-24 4:46 ` Maged Michael
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