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=-1.0 required=5.0 tests=MAILING_LIST_MULTI, RCVD_IN_MSPIKE_H2 autolearn=ham autolearn_force=no version=3.4.4 Received: (qmail 2056 invoked from network); 3 Oct 2022 23:05:22 -0000 Received: from second.openwall.net (193.110.157.125) by inbox.vuxu.org with ESMTPUTF8; 3 Oct 2022 23:05:22 -0000 Received: (qmail 30178 invoked by uid 550); 3 Oct 2022 23:05:20 -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 30146 invoked from network); 3 Oct 2022 23:05:18 -0000 Date: Mon, 3 Oct 2022 19:05:06 -0400 From: Rich Felker To: musl@lists.openwall.com, Alexey Izbyshev Message-ID: <20221003230505.GH29905@brightrain.aerifal.cx> References: <20220919152937.GQ9709@brightrain.aerifal.cx> <20221003132615.GF2158779@port70.net> <20221003212705.GG2158779@port70.net> <20221003225416.GG29905@brightrain.aerifal.cx> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20221003225416.GG29905@brightrain.aerifal.cx> User-Agent: Mutt/1.5.21 (2010-09-15) Subject: Re: [musl] Illegal killlock skipping when transitioning to single-threaded state On Mon, Oct 03, 2022 at 06:54:17PM -0400, Rich Felker wrote: > On Mon, Oct 03, 2022 at 11:27:05PM +0200, Szabolcs Nagy wrote: > > * Szabolcs Nagy [2022-10-03 15:26:15 +0200]: > > > > > * Alexey Izbyshev [2022-10-03 09:16:03 +0300]: > > > > On 2022-09-19 18:29, Rich Felker wrote: > > > > > On Wed, Sep 07, 2022 at 03:46:53AM +0300, Alexey Izbyshev wrote: > > > ... > > > > > > Reordering the "libc.need_locks = -1" assignment and > > > > > > UNLOCK(E->killlock) and providing a store barrier between them > > > > > > should fix the issue. > > > > > > > > > > I think this all sounds correct. I'm not sure what you mean by a store > > > > > barrier between them, since all lock and unlock operations are already > > > > > full barriers. > > > > > > > > > > > > > Before sending the report I tried to infer the intended ordering semantics > > > > of LOCK/UNLOCK by looking at their implementations. For AArch64, I didn't > > > > see why they would provide a full barrier (my reasoning is below), so I > > > > concluded that probably acquire/release semantics was intended in general > > > > and suggested an extra store barrier to prevent hoisting of "libc.need_locks > > > > = -1" store spelled after UNLOCK(E->killlock) back into the critical > > > > section. > > > > > > > > UNLOCK is implemented via a_fetch_add(). On AArch64, it is a simple > > > > a_ll()/a_sc() loop without extra barriers, and a_ll()/a_sc() are implemented > > > > via load-acquire/store-release instructions. Therefore, if we consider a > > > > LOCK/UNLOCK critical section containing only plain loads and stores, (a) any > > > > such memory access can be reordered with the initial ldaxr in UNLOCK, and > > > > (b) any plain load following UNLOCK can be reordered with stlxr (assuming > > > > the processor predicts that stlxr succeeds), and further, due to (a), with > > > > any memory access inside the critical section. Therefore, UNLOCK is not full > > > > barrier. Is this right? > > > > > > i dont think this is right. > > > > > > i think i was wrong and you are right. > > > > so with your suggested swap of UNLOCK(killlock) and need_locks=-1 and > > starting with 'something == 0' the exiting E and remaining R threads: > > > > E:something=1 // protected by killlock > > E:UNLOCK(killlock) > > E:need_locks=-1 > > > > R:LOCK(unrelated) // reads need_locks == -1 > > R:need_locks=0 > > R:UNLOCK(unrelated) > > R:LOCK(killlock) // does not lock > > R:read something // can it be 0 ? > > > > and here something can be 0 (ie. not protected by killlock) on aarch64 > > because > > > > T1 > > something=1 > > ldaxr ... killlock > > stlxr ... killlock > > need_locks=-1 > > > > T2 > > x=need_locks > > ldaxr ... unrelated > > stlxr ... unrelated > > y=something > > > > can end with x==-1 and y==0. > > > > and to fix it, both a_fetch_add and a_cas need an a_barrier. > > > > i need to think how to support such lock usage on aarch64 > > without adding too many dmb. > > I don't really understand this, but FWIW gcc emits > > ldxr > ... > stlxr > ... > dmb ish > > for __sync_val_compare_and_swap. So this is probably the right thing > we should have. And it seems to match what the kernel folks discussed > here: > > http://lists.infradead.org/pipermail/linux-arm-kernel/2014-February/229588.html > > I wondered if there are similar issues for any others archs which need > review, but it looks like all the other llsc archs have explicit > pre/post barriers defined. Actually I don't understand what's going on with cmpxchg there. The patch I linked has it using ldxr/stxr (not stlxr) for cmpxchg. There's some follow-up in the thread I don't understand, about the case where the cas fails, but we already handle that by doing an explicit barrier in that case. This stuff is a poorly-documented mess. >_< Rich