From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:58546) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ddfyS-0006t8-PL for qemu-devel@nongnu.org; Fri, 04 Aug 2017 13:01:21 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ddfyO-0001KD-81 for qemu-devel@nongnu.org; Fri, 04 Aug 2017 13:01:20 -0400 Received: from mx1.redhat.com ([209.132.183.28]:53948) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1ddfyN-0001JK-VB for qemu-devel@nongnu.org; Fri, 04 Aug 2017 13:01:16 -0400 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 2FF3E80E75 for ; Fri, 4 Aug 2017 17:01:14 +0000 (UTC) Date: Fri, 4 Aug 2017 18:01:10 +0100 From: "Dr. David Alan Gilbert" Message-ID: <20170804170110.GD2087@work-vm> References: <20170804161452.17413-1-pbonzini@redhat.com> <20170804161452.17413-2-pbonzini@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170804161452.17413-2-pbonzini@redhat.com> Subject: Re: [Qemu-devel] [PATCH 1/2] rcu: completely disable pthread_atfork callbacks as soon as possible List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: qemu-devel@nongnu.org * Paolo Bonzini (pbonzini@redhat.com) wrote: > Because of -daemonize, system mode QEMU sometimes needs to fork() and > keep RCU enabled in the child. However, there is a possible deadlock > with synchronize_rcu: > > - the CPU thread is inside a RCU critical section and wants to take > the BQL in order to do MMIO > > - the monitor thread, which is owning the BQL, calls rcu_init_lock > which tries to take the rcu_sync_lock > > - the call_rcu thread has taken rcu_sync_lock in synchronize_rcu, but > synchronize_rcu needs the CPU thread to end the critical section > before returning. > > This cannot happen for user-mode emulation, because it does not have > a BQL. > > To fix it, assume that system mode QEMU only forks in preparation for > exec (except when daemonizing) and disable pthread_atfork as soon as > the double fork has happened. > > Reported-by: David Gilbert > Signed-off-by: Paolo Bonzini That seems to fix that issue (not tested it any more than that though), so for fixing that issue: Tested-by: Dr. David Alan Gilbert > --- > include/qemu/rcu.h | 6 ++++++ > util/rcu.c | 20 ++++++++++++++++++++ > vl.c | 1 + > 3 files changed, 27 insertions(+) > > diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h > index 83ae2808be..c0da9907e8 100644 > --- a/include/qemu/rcu.h > +++ b/include/qemu/rcu.h > @@ -105,6 +105,12 @@ extern void synchronize_rcu(void); > */ > extern void rcu_register_thread(void); > extern void rcu_unregister_thread(void); > + > +/* > + * Support for fork(). fork() support is enabled at startup. > + */ > +extern void rcu_enable_atfork(void); > +extern void rcu_disable_atfork(void); > extern void rcu_after_fork(void); > > struct rcu_head; > diff --git a/util/rcu.c b/util/rcu.c > index 9adc5e4a36..2142ddd93b 100644 > --- a/util/rcu.c > +++ b/util/rcu.c > @@ -318,15 +318,35 @@ static void rcu_init_complete(void) > rcu_register_thread(); > } > > +static int atfork_depth = 1; > + > +void rcu_enable_atfork(void) > +{ > + atfork_depth++; > +} > + > +void rcu_disable_atfork(void) > +{ > + atfork_depth--; > +} > + > #ifdef CONFIG_POSIX > static void rcu_init_lock(void) > { > + if (atfork_depth < 1) { > + return; > + } > + > qemu_mutex_lock(&rcu_sync_lock); > qemu_mutex_lock(&rcu_registry_lock); > } > > static void rcu_init_unlock(void) > { > + if (atfork_depth < 1) { > + return; > + } > + > qemu_mutex_unlock(&rcu_registry_lock); > qemu_mutex_unlock(&rcu_sync_lock); > } > diff --git a/vl.c b/vl.c > index 99fcfa0442..8967115514 100644 > --- a/vl.c > +++ b/vl.c > @@ -4121,6 +4121,7 @@ int main(int argc, char **argv, char **envp) > set_memory_options(&ram_slots, &maxram_size, machine_class); > > os_daemonize(); > + rcu_disable_atfork(); > > if (pid_file && qemu_create_pidfile(pid_file) != 0) { > error_report("could not acquire pid file: %s", strerror(errno)); > -- > 2.13.3 > > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK