From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6C6A7C433EF for ; Tue, 9 Nov 2021 18:40:21 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 0370961177 for ; Tue, 9 Nov 2021 18:40:20 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 0370961177 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kaod.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=nongnu.org Received: from localhost ([::1]:44438 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mkW2d-0008JC-My for qemu-devel@archiver.kernel.org; Tue, 09 Nov 2021 13:40:19 -0500 Received: from eggs.gnu.org ([209.51.188.92]:55970) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mkVy5-0003Rm-4d for qemu-devel@nongnu.org; Tue, 09 Nov 2021 13:35:37 -0500 Received: from us-smtp-delivery-44.mimecast.com ([207.211.30.44]:41696) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mkVy3-0001NG-D9 for qemu-devel@nongnu.org; Tue, 09 Nov 2021 13:35:36 -0500 Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-283-aTq5WQk6NJehmB_xPE5QGA-1; Tue, 09 Nov 2021 13:35:31 -0500 X-MC-Unique: aTq5WQk6NJehmB_xPE5QGA-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 996FF87D541; Tue, 9 Nov 2021 18:35:30 +0000 (UTC) Received: from bahia.redhat.com (unknown [10.39.194.211]) by smtp.corp.redhat.com (Postfix) with ESMTP id 1A75660D30; Tue, 9 Nov 2021 18:35:28 +0000 (UTC) From: Greg Kurz To: qemu-devel@nongnu.org Subject: [PATCH v4 2/2] accel/tcg: Register a force_rcu notifier Date: Tue, 9 Nov 2021 19:35:23 +0100 Message-Id: <20211109183523.47726-3-groug@kaod.org> In-Reply-To: <20211109183523.47726-1-groug@kaod.org> References: <20211109183523.47726-1-groug@kaod.org> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=groug@kaod.org X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: kaod.org Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=WINDOWS-1252 Received-SPF: softfail client-ip=207.211.30.44; envelope-from=groug@kaod.org; helo=us-smtp-delivery-44.mimecast.com X-Spam_score_int: -18 X-Spam_score: -1.9 X-Spam_bar: - X-Spam_report: (-1.9 / 5.0 requ) BAYES_00=-1.9, RCVD_IN_DNSWL_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_SOFTFAIL=0.665 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Eduardo Habkost , Richard Henderson , Greg Kurz , qemu-stable@nongnu.org, Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" A TCG vCPU doing a busy loop systematicaly hangs the QEMU monitor if the user passes 'device_add' without argument. This is because drain_cpu_all() which is called from qmp_device_add() cannot return if readers don't exit read-side critical sections. That is typically what busy-looping TCG vCPUs do: int cpu_exec(CPUState *cpu) { [...] rcu_read_lock(); [...] while (!cpu_handle_exception(cpu, &ret)) { // Busy loop keeps vCPU here } [...] rcu_read_unlock(); return ret; } For MTTCG, have all vCPU threads register a force_rcu notifier that will kick them out of the loop using async_run_on_cpu(). The notifier is called with the rcu_registry_lock mutex held, using async_run_on_cpu() ensures there are no deadlocks. For RR, a single thread runs all vCPUs. Just register a single notifier that kicks the current vCPU to the next one. For MTTCG: Suggested-by: Paolo Bonzini For RR: Suggested-by: Richard Henderson Fixes: 7bed89958bfb ("device_core: use drain_call_rcu in in qmp_device_add"= ) Resolves: https://gitlab.com/qemu-project/qemu/-/issues/650 Signed-off-by: Greg Kurz --- accel/tcg/tcg-accel-ops-mttcg.c | 26 ++++++++++++++++++++++++++ accel/tcg/tcg-accel-ops-rr.c | 10 ++++++++++ 2 files changed, 36 insertions(+) diff --git a/accel/tcg/tcg-accel-ops-mttcg.c b/accel/tcg/tcg-accel-ops-mttc= g.c index 847d2079d21f..29632bd4c0af 100644 --- a/accel/tcg/tcg-accel-ops-mttcg.c +++ b/accel/tcg/tcg-accel-ops-mttcg.c @@ -28,6 +28,7 @@ #include "sysemu/tcg.h" #include "sysemu/replay.h" #include "qemu/main-loop.h" +#include "qemu/notify.h" #include "qemu/guest-random.h" #include "exec/exec-all.h" #include "hw/boards.h" @@ -35,6 +36,26 @@ #include "tcg-accel-ops.h" #include "tcg-accel-ops-mttcg.h" =20 +typedef struct MttcgForceRcuNotifier { + Notifier notifier; + CPUState *cpu; +} MttcgForceRcuNotifier; + +static void do_nothing(CPUState *cpu, run_on_cpu_data d) +{ +} + +static void mttcg_force_rcu(Notifier *notify, void *data) +{ + CPUState *cpu =3D container_of(notify, MttcgForceRcuNotifier, notifier= )->cpu; + + /* + * Called with rcu_registry_lock held, using async_run_on_cpu() ensure= s + * that there are no deadlocks. + */ + async_run_on_cpu(cpu, do_nothing, RUN_ON_CPU_NULL); +} + /* * In the multi-threaded case each vCPU has its own thread. The TLS * variable current_cpu can be used deep in the code to find the @@ -43,12 +64,16 @@ =20 static void *mttcg_cpu_thread_fn(void *arg) { + MttcgForceRcuNotifier force_rcu; CPUState *cpu =3D arg; =20 assert(tcg_enabled()); g_assert(!icount_enabled()); =20 rcu_register_thread(); + force_rcu.notifier.notify =3D mttcg_force_rcu; + force_rcu.cpu =3D cpu; + rcu_add_force_rcu_notifier(&force_rcu.notifier); tcg_register_thread(); =20 qemu_mutex_lock_iothread(); @@ -100,6 +125,7 @@ static void *mttcg_cpu_thread_fn(void *arg) =20 tcg_cpus_destroy(cpu); qemu_mutex_unlock_iothread(); + rcu_remove_force_rcu_notifier(&force_rcu.notifier); rcu_unregister_thread(); return NULL; } diff --git a/accel/tcg/tcg-accel-ops-rr.c b/accel/tcg/tcg-accel-ops-rr.c index a5fd26190e20..bf59f53dbc2c 100644 --- a/accel/tcg/tcg-accel-ops-rr.c +++ b/accel/tcg/tcg-accel-ops-rr.c @@ -28,6 +28,7 @@ #include "sysemu/tcg.h" #include "sysemu/replay.h" #include "qemu/main-loop.h" +#include "qemu/notify.h" #include "qemu/guest-random.h" #include "exec/exec-all.h" =20 @@ -133,6 +134,11 @@ static void rr_deal_with_unplugged_cpus(void) } } =20 +static void rr_force_rcu(Notifier *notify, void *data) +{ + rr_kick_next_cpu(); +} + /* * In the single-threaded case each vCPU is simulated in turn. If * there is more than a single vCPU we create a simple timer to kick @@ -143,10 +149,13 @@ static void rr_deal_with_unplugged_cpus(void) =20 static void *rr_cpu_thread_fn(void *arg) { + Notifier force_rcu; CPUState *cpu =3D arg; =20 assert(tcg_enabled()); rcu_register_thread(); + force_rcu.notify =3D rr_force_rcu; + rcu_add_force_rcu_notifier(&force_rcu); tcg_register_thread(); =20 qemu_mutex_lock_iothread(); @@ -255,6 +264,7 @@ static void *rr_cpu_thread_fn(void *arg) rr_deal_with_unplugged_cpus(); } =20 + rcu_remove_force_rcu_notifier(&force_rcu); rcu_unregister_thread(); return NULL; } --=20 2.31.1