From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:46038) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bIGwI-00005C-Rc for qemu-devel@nongnu.org; Wed, 29 Jun 2016 10:58:07 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bIGwE-0006eh-L1 for qemu-devel@nongnu.org; Wed, 29 Jun 2016 10:58:05 -0400 Received: from mail-lf0-x244.google.com ([2a00:1450:4010:c07::244]:36014) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bIGwE-0006eZ-8P for qemu-devel@nongnu.org; Wed, 29 Jun 2016 10:58:02 -0400 Received: by mail-lf0-x244.google.com with SMTP id a2so5351726lfe.3 for ; Wed, 29 Jun 2016 07:58:02 -0700 (PDT) References: <87k2hbdmxt.fsf@linaro.org> From: Sergey Fedorov Message-ID: <5773E1F7.6000709@gmail.com> Date: Wed, 29 Jun 2016 17:57:59 +0300 MIME-Version: 1.0 In-Reply-To: <87k2hbdmxt.fsf@linaro.org> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Subject: Re: [Qemu-devel] [RFC 4/8] linux-user: Rework exclusive operation mechanism List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: =?UTF-8?Q?Alex_Benn=c3=a9e?= , Sergey Fedorov Cc: qemu-devel@nongnu.org, Riku Voipio , patches@linaro.org On 27/06/16 12:02, Alex Bennée wrote: > Sergey Fedorov writes: > >> From: Sergey Fedorov >> (snip) >> diff --git a/linux-user/main.c b/linux-user/main.c >> index b9a4e0ea45ac..485336f78b8f 100644 >> --- a/linux-user/main.c >> +++ b/linux-user/main.c >> @@ -111,7 +111,8 @@ static pthread_mutex_t cpu_list_mutex = PTHREAD_MUTEX_INITIALIZER; >> static pthread_mutex_t exclusive_lock = PTHREAD_MUTEX_INITIALIZER; >> static pthread_cond_t exclusive_cond = PTHREAD_COND_INITIALIZER; >> static pthread_cond_t exclusive_resume = PTHREAD_COND_INITIALIZER; >> -static int pending_cpus; >> +static bool exclusive_pending; >> +static int tcg_pending_cpus; > I'm not sure you need to re-name to tcg_pending_cpus as TCG is implied > for linux-user. Also they are not really CPUs (although we are using the > CPU structure for each running thread). I'm not sure if there is a > neater way to make the distinction clear. How about 'tcg_pending_threads'? It is going to be used in system-mode soon, so I'd like to keep "tcg_" prefix. > >> /* Make sure everything is in a consistent state for calling fork(). */ >> void fork_start(void) >> @@ -133,7 +134,8 @@ void fork_end(int child) >> QTAILQ_REMOVE(&cpus, cpu, node); >> } >> } >> - pending_cpus = 0; >> + tcg_pending_cpus = 0; >> + exclusive_pending = false; >> pthread_mutex_init(&exclusive_lock, NULL); >> pthread_mutex_init(&cpu_list_mutex, NULL); >> pthread_cond_init(&exclusive_cond, NULL); >> @@ -150,7 +152,7 @@ void fork_end(int child) >> must be held. */ >> static inline void exclusive_idle(void) >> { >> - while (pending_cpus) { >> + while (exclusive_pending) { >> pthread_cond_wait(&exclusive_resume, &exclusive_lock); >> } >> } >> @@ -164,15 +166,14 @@ static inline void start_exclusive(void) >> pthread_mutex_lock(&exclusive_lock); >> exclusive_idle(); >> >> - pending_cpus = 1; >> + exclusive_pending = true; >> /* Make all other cpus stop executing. */ >> CPU_FOREACH(other_cpu) { >> if (other_cpu->running) { >> - pending_cpus++; >> cpu_exit(other_cpu); >> } >> } >> - if (pending_cpus > 1) { >> + while (tcg_pending_cpus) { >> pthread_cond_wait(&exclusive_cond, &exclusive_lock); >> } >> } >> @@ -180,7 +181,7 @@ static inline void start_exclusive(void) >> /* Finish an exclusive operation. */ >> static inline void __attribute__((unused)) end_exclusive(void) >> { >> - pending_cpus = 0; >> + exclusive_pending = false; >> pthread_cond_broadcast(&exclusive_resume); >> pthread_mutex_unlock(&exclusive_lock); >> } >> @@ -191,6 +192,7 @@ static inline void cpu_exec_start(CPUState *cpu) >> pthread_mutex_lock(&exclusive_lock); >> exclusive_idle(); >> cpu->running = true; >> + tcg_pending_cpus++; > These aren't TLS variables so shouldn't we be ensuring all access is atomic? It is protected by 'exclusive_lock'. > >> pthread_mutex_unlock(&exclusive_lock); >> } >> >> @@ -199,11 +201,9 @@ static inline void cpu_exec_end(CPUState *cpu) >> { >> pthread_mutex_lock(&exclusive_lock); >> cpu->running = false; >> - if (pending_cpus > 1) { >> - pending_cpus--; >> - if (pending_cpus == 1) { >> - pthread_cond_signal(&exclusive_cond); >> - } >> + tcg_pending_cpus--; >> + if (!tcg_pending_cpus) { >> + pthread_cond_broadcast(&exclusive_cond); >> } > Couldn't two threads race to -1 here? See comment above. Kind regards, Sergey > >> exclusive_idle(); >> pthread_mutex_unlock(&exclusive_lock); > > -- > Alex Bennée