Date: Wed, 21 Jul 2021 00:26:02 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Andriy Gapon <avg@freebsd.org> Cc: Alexander Motin <mav@freebsd.org>, 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: <YPc/aip3wJVvxOCy@kib.kiev.ua> In-Reply-To: <df8b37ed-d115-4361-d8e1-aa80a4a601af@FreeBSD.org> References: <202107201730.16KHUXPM057550@gitrepo.freebsd.org> <YPcmvk924TKbOMu9@kib.kiev.ua> <df8b37ed-d115-4361-d8e1-aa80a4a601af@FreeBSD.org>
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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 <mav@FreeBSD.org> > > > AuthorDate: 2021-07-20 17:15:08 +0000 > > > Commit: Alexander Motin <mav@FreeBSD.org> > > > 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 <stdbool.h> > > > #include <stdlib.h> > > > #include <syslog.h> > > > +#include <machine/atomic.h> > > > #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 <avg@FreeBSD.org> > AuthorDate: Mon Apr 5 17:33:07 2021 +0300 > Commit: Andriy Gapon <avg@FreeBSD.org> > 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 <sys/param.h> > #include <sys/sysctl.h> > #include <assert.h> > +#include <pthread.h> > #include <stdbool.h> > #include <stdlib.h> > #include <syslog.h> > #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.
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