From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on inbox.vuxu.org X-Spam-Level: X-Spam-Status: No, score=-3.3 required=5.0 tests=MAILING_LIST_MULTI, RCVD_IN_DNSWL_MED,RCVD_IN_MSPIKE_H3,RCVD_IN_MSPIKE_WL autolearn=ham autolearn_force=no version=3.4.4 Received: (qmail 6926 invoked from network); 23 Nov 2020 14:53:46 -0000 Received: from mother.openwall.net (195.42.179.200) by inbox.vuxu.org with ESMTPUTF8; 23 Nov 2020 14:53:46 -0000 Received: (qmail 30388 invoked by uid 550); 23 Nov 2020 14:53:42 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Reply-To: musl@lists.openwall.com Received: (qmail 30361 invoked from network); 23 Nov 2020 14:53:42 -0000 Date: Mon, 23 Nov 2020 09:53:29 -0500 From: Rich Felker To: Jonathan Wakely Cc: Florian Weimer , =?utf-8?B?0JDRgNGB0LXQvdC40Lk=?= , musl@lists.openwall.com Message-ID: <20201123145329.GU534@brightrain.aerifal.cx> References: <1605849917.737458858@f540.i.mail.ru> <1605941217.735966206@f110.i.mail.ru> <20201121155128.GN534@brightrain.aerifal.cx> <1606070615.957905591@f395.i.mail.ru> <20201122191106.GO534@brightrain.aerifal.cx> <87mtz99owk.fsf@mid.deneb.enyo.de> <20201122192824.GQ534@brightrain.aerifal.cx> <20201123114124.GP1312820@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20201123114124.GP1312820@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) Subject: Re: [musl] Mutexes are not unlocking On Mon, Nov 23, 2020 at 11:41:24AM +0000, Jonathan Wakely wrote: > On 22/11/20 14:28 -0500, Rich Felker wrote: > >On Sun, Nov 22, 2020 at 08:23:23PM +0100, Florian Weimer wrote: > >>* Rich Felker: > >> > >>> On Sun, Nov 22, 2020 at 09:43:35PM +0300, Арсений wrote: > >>>> > >>>> Hello, > >>>> The problem is that mutex is not got unlocked after the first unlock(). > >>>>   > >>>> libstdc++ uses a wrapper for pthread called gthreads. This wrapper > >>>> checks for the state of the mutex system. For > >>>> example, pthread_mutex_unlock() is called in a following way: > >>>>   > >>>> static inline int > >>>> __gthread_mutex_unlock (__gthread_mutex_t *__mutex) > >>>> { > >>>> if (__gthread_active_p ()) > >>>> return __gthrw_(pthread_mutex_unlock) (__mutex); > >>>> else > >>>> return 0; > >>>> } > >>> > >>> Yes. This code is invalid (it misinterprets weak symbol information to > >>> draw incorrect conclusions about whether threads may be in use) and > >>> thus is disabled in builds of gcc/libstdc++ targeting musl-based > >>> systems. GCC and glibc-based distro folks mostly don't care because > >>> it only breaks static linking, but some of them actually hack gcc's > >>> libpthread.a into one giant .o file to work around the problem rather > >>> than fixing this in gcc... > >> > >>GCC 11 has a fix (if used along with glibc 2.32), but I wonder if it's > > The GCC 11 change isn't used for mutexes. I use __libc_single_threaded > for deciding whether to use atomics or plain non-atomic ops for > ref-count updates. But for actions like mutex locks I don't use it, > because it would do the wrong thing for code like: > > int main() { > std::mutex m; > std::lock_guard l(m); > std::thread t( []{} }; > t.join(); > } > > In this program __libc_single_threaded is true when we construct the > lock_guard, but false when it is destroyed at the end of the block. If > we checked __libc_single_threaded for mutex locking/unlocking then we > would be inconsistent here, we would elide the lock but try to unlock > a mutex that was never locked, which would be UB. > > Whatever condition we check to decide whether to call the pthreads > function needs to give the same result for the lock and unlock. That > condition is currently __gthread_active_p, which checks if a symbol is > weak. > > >>going to run into a similar issue because inlined code from older GCC > >>versions uses a diverging version of the check. > > No, everything uses the same check. GCC 11 didn't change anything for > mutexes. > > >>Jonathan, more context is here: > >> > >> > > > >Uhg, that's a really nasty problem. Is __gthread_active_p() also > >inlined? Or can it be given a definition to mitigate the problem? > > It's inlined, and making it a non-inline PLT call would negate some of > the point of using it (it's there mostly as an optimization). > > But I don't think inlining it is a problem, because its definition > hasn't changed. > > I think the right thing to do is a new configuration which completely > avoids using weak symbols in gthreads, and just calls the pthread > symbols directly. A future version of glibc is going to move pthreads > symbols into libc anyway: > https://sourceware.org/legacy-ml/libc-alpha/2019-10/msg00080.html > After that, the __gthread_active_p condition is just a wasted branch, > because it will be unconditionally true: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89714 The potential problem is that removing the condition makes an inconsistency with binaries where the condition was already inlined. > If the musl libstdc++ removes the use of __gthread_active_p, but code > compiled against a glibc libstdc++ inlines the __gthread_active_p > check, then it seems to me that musl (or the musl libstdc++) needs to > ensure that the inlined __gthread_active_p will return true. It can do > that by providing a non-weak symbol called __pthread_key_create (the > symbol won't be called, it just has to exist). The target files for -musl tuples disable the hack entirely by adding t-gthr-noweak and -DGTHREAD_USE_WEAK=0 where appropriate. So there's no issue here. OP's problem was trying to make glibc-linked binaries and use them with musl's very limited glibc-ABI-compat capabilities, which is not recommended; it's only intended as an aid in running binaryware (esp shared libraries) you can't build/get native versions of. Rich