From owner-dev-commits-src-main@freebsd.org Tue Jul 20 21:26:11 2021 Return-Path: Delivered-To: dev-commits-src-main@mailman.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.nyi.freebsd.org (Postfix) with ESMTP id C76A56531F5; Tue, 20 Jul 2021 21:26:11 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from kib.kiev.ua (kib.kiev.ua [IPv6:2001:470:d5e7:1::1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 4GTsGC3dHkz4jP1; Tue, 20 Jul 2021 21:26:11 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from tom.home (kib@localhost [127.0.0.1]) by kib.kiev.ua (8.16.1/8.16.1) with ESMTPS id 16KLQ2iV066872 (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NO); Wed, 21 Jul 2021 00:26:06 +0300 (EEST) (envelope-from kostikbel@gmail.com) DKIM-Filter: OpenDKIM Filter v2.10.3 kib.kiev.ua 16KLQ2iV066872 Received: (from kostik@localhost) by tom.home (8.16.1/8.16.1/Submit) id 16KLQ2Xi066871; Wed, 21 Jul 2021 00:26:02 +0300 (EEST) (envelope-from kostikbel@gmail.com) X-Authentication-Warning: tom.home: kostik set sender to kostikbel@gmail.com using -f Date: Wed, 21 Jul 2021 00:26:02 +0300 From: Konstantin Belousov To: Andriy Gapon Cc: Alexander Motin , src-committers@freebsd.org, dev-commits-src-all@freebsd.org, dev-commits-src-main@freebsd.org Subject: Re: git: 28d70deaafa6 - main - Fix race between first rand(3) calls. Message-ID: References: <202107201730.16KHUXPM057550@gitrepo.freebsd.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Spam-Status: No, score=-1.0 required=5.0 tests=ALL_TRUSTED,BAYES_00, DKIM_ADSP_CUSTOM_MED,FORGED_GMAIL_RCVD,FREEMAIL_FROM, NML_ADSP_CUSTOM_MED autolearn=no autolearn_force=no version=3.4.5 X-Spam-Checker-Version: SpamAssassin 3.4.5 (2021-03-20) on tom.home X-Rspamd-Queue-Id: 4GTsGC3dHkz4jP1 X-Spamd-Bar: ---- Authentication-Results: mx1.freebsd.org; none X-Spamd-Result: default: False [-4.00 / 15.00]; REPLY(-4.00)[] X-BeenThere: dev-commits-src-main@freebsd.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: Commit messages for the main branch of the src repository List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 20 Jul 2021 21:26:11 -0000 On Wed, Jul 21, 2021 at 12:10:29AM +0300, Andriy Gapon wrote: > On 20/07/2021 22:40, Konstantin Belousov wrote: > > On Tue, Jul 20, 2021 at 05:30:33PM +0000, Alexander Motin wrote: > > > The branch main has been updated by mav: > > > > > > URL: https://cgit.FreeBSD.org/src/commit/?id=28d70deaafa62c5d1602de5272c0aad0fcca8aff > > > > > > commit 28d70deaafa62c5d1602de5272c0aad0fcca8aff > > > Author: Alexander Motin > > > AuthorDate: 2021-07-20 17:15:08 +0000 > > > Commit: Alexander Motin > > > CommitDate: 2021-07-20 17:30:28 +0000 > > > > > > Fix race between first rand(3) calls. > > > Before this patch there was a chance for thread that called rand(3) > > > slightly later to see rand3_state already allocated, but not yet > > > initialized. While this API is not expected to be thread-safe, it > > > is not expected to crash. ztest on 64-thread system reproduced it > > > reliably for me. > > > MFC after: 1 month > > > --- > > > lib/libc/stdlib/rand.c | 9 +++++++-- > > > 1 file changed, 7 insertions(+), 2 deletions(-) > > > > > > diff --git a/lib/libc/stdlib/rand.c b/lib/libc/stdlib/rand.c > > > index bddb0f040302..353f59349e1d 100644 > > > --- a/lib/libc/stdlib/rand.c > > > +++ b/lib/libc/stdlib/rand.c > > > @@ -44,6 +44,7 @@ __FBSDID("$FreeBSD$"); > > > #include > > > #include > > > #include > > > +#include > > > #include "un-namespace.h" > > > #include "random.h" > > > @@ -68,11 +69,15 @@ static struct __random_state *rand3_state; > > > static void > > > initialize_rand3(void) > > > { > > > + struct __random_state *state; > > > int error; > > > - rand3_state = allocatestate(TYPE_3); > > > - error = initstate_r(rand3_state, 1, rand3_state->rst_randtbl, BREAK_3); > > > + state = allocatestate(TYPE_3); > > > + error = initstate_r(state, 1, state->rst_randtbl, BREAK_3); > > > assert(error == 0); > > > + if (!atomic_cmpset_rel_ptr((volatile uintptr_t *)&rand3_state, > > > + (uintptr_t)NULL, (uintptr_t)state)) > > > + free(state); > > For this to have effect on less ordered architectures (AKA non-x86), at > > least reads of rand3_state should be atomic_load_acq(). > > > > But consider using _once() in libc, which takes care about ordering/threads. > > FWIW, I've been using the following patch locally. > And the reason for the patched is discussed here: > https://www.mail-archive.com/freebsd-current@freebsd.org/msg182365.html > > commit cc38290d14a5bbab64e419a2d6d8c34f61840829 > Author: Andriy Gapon > AuthorDate: Mon Apr 5 17:33:07 2021 +0300 > Commit: Andriy Gapon > CommitDate: Tue Jun 8 10:34:28 2021 +0300 > > [tmp?] make rand/srand initialization thread safe > > But rand(3) still remains non-thread-safe in general. > This is only to prevent an initialization time crash in ztest. > Probably should be fixed there, e.g. by using a different random number > API or by adding some locking. > > diff --git a/lib/libc/stdlib/rand.c b/lib/libc/stdlib/rand.c > index bddb0f040302..0d3ed2230e10 100644 > --- a/lib/libc/stdlib/rand.c > +++ b/lib/libc/stdlib/rand.c > @@ -41,11 +41,13 @@ __FBSDID("$FreeBSD$"); > #include > #include > #include > +#include > #include > #include > #include > #include "un-namespace.h" > > +#include "libc_private.h" > #include "random.h" > > /* > @@ -64,6 +66,7 @@ __FBSDID("$FreeBSD$"); > * the advantage of being the one already in the tree. > */ > static struct __random_state *rand3_state; > +static pthread_once_t __random_state_once = PTHREAD_ONCE_INIT; > > static void > initialize_rand3(void) > @@ -78,16 +81,14 @@ initialize_rand3(void) > int > rand(void) > { > - if (rand3_state == NULL) > - initialize_rand3(); > + _once(&__random_state_once, initialize_rand3); > return ((int)random_r(rand3_state)); > } > > void > srand(unsigned seed) > { > - if (rand3_state == NULL) > - initialize_rand3(); > + _once(&__random_state_once, initialize_rand3); > srandom_r(rand3_state, seed); > } > Right, this should be it.