From nobody Tue May 21 23:09:31 2024 Delivered-To: importer@patchew.org Authentication-Results: mx.zohomail.com; spf=pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org Return-Path: Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) by mx.zohomail.com with SMTPS id 1634623091199697.5441350118699; Mon, 18 Oct 2021 22:58:11 -0700 (PDT) Received: from localhost ([::1]:54270 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mci8Y-0001VL-6x for importer@patchew.org; Tue, 19 Oct 2021 01:58:10 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:37684) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mci7C-0007OS-4J for qemu-devel@nongnu.org; Tue, 19 Oct 2021 01:56:46 -0400 Received: from us-smtp-delivery-44.mimecast.com ([205.139.111.44]:31751) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mci78-0002yd-Qb for qemu-devel@nongnu.org; Tue, 19 Oct 2021 01:56:45 -0400 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-158-zLUNfAYuOqKlW5vRYcvR4Q-1; Tue, 19 Oct 2021 01:56:37 -0400 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id C2E209126D; Tue, 19 Oct 2021 05:56:36 +0000 (UTC) Received: from bahia.redhat.com (unknown [10.39.192.173]) by smtp.corp.redhat.com (Postfix) with ESMTP id 4C4E119C59; Tue, 19 Oct 2021 05:56:35 +0000 (UTC) X-MC-Unique: zLUNfAYuOqKlW5vRYcvR4Q-1 From: Greg Kurz To: qemu-devel@nongnu.org Subject: [PATCH v2 1/2] rcu: Introduce force_rcu notifier Date: Tue, 19 Oct 2021 07:56:31 +0200 Message-Id: <20211019055632.252879-2-groug@kaod.org> In-Reply-To: <20211019055632.252879-1-groug@kaod.org> References: <20211019055632.252879-1-groug@kaod.org> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 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 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: softfail client-ip=205.139.111.44; envelope-from=groug@kaod.org; helo=us-smtp-delivery-44.mimecast.com X-Spam_score_int: 0 X-Spam_score: -0.0 X-Spam_bar: / X-Spam_report: (-0.0 / 5.0 requ) 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: 2.1.23 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+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZM-MESSAGEID: 1634623093114100007 Content-Type: text/plain; charset="utf-8" The drain_rcu_call() function can be blocked as long as an RCU reader stays in a read-side critical section. This is typically what happens when a TCG vCPU is executing a busy loop. It can deadlock the QEMU monitor as reported in https://gitlab.com/qemu-project/qemu/-/issues/650 . This can be avoided by allowing drain_rcu_call() to enforce an RCU grace period. Since each reader might need to do specific actions to end a read-side critical section, do it with notifiers. Prepare ground for this by adding a notifier list to the RCU reader struct and use it in wait_for_readers() if drain_rcu_call() is in progress. An API is added for readers to register their notifiers. This is largely based on a draft from Paolo Bonzini. Suggested-by: Paolo Bonzini Signed-off-by: Greg Kurz --- include/qemu/rcu.h | 16 ++++++++++++++++ util/rcu.c | 22 +++++++++++++++++++++- 2 files changed, 37 insertions(+), 1 deletion(-) diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h index 515d327cf11c..d8c4fd8686b4 100644 --- a/include/qemu/rcu.h +++ b/include/qemu/rcu.h @@ -27,6 +27,7 @@ #include "qemu/thread.h" #include "qemu/queue.h" #include "qemu/atomic.h" +#include "qemu/notify.h" #include "qemu/sys_membarrier.h" =20 #ifdef __cplusplus @@ -66,6 +67,14 @@ struct rcu_reader_data { =20 /* Data used for registry, protected by rcu_registry_lock */ QLIST_ENTRY(rcu_reader_data) node; + + /* + * NotifierList used to force an RCU grace period. Accessed under + * rcu_registry_lock. Note that the notifier is called _outside_ + * the thread! + */ + NotifierList force_rcu; + void *force_rcu_data; }; =20 extern __thread struct rcu_reader_data rcu_reader; @@ -180,6 +189,13 @@ G_DEFINE_AUTOPTR_CLEANUP_FUNC(RCUReadAuto, rcu_read_au= to_unlock) #define RCU_READ_LOCK_GUARD() \ g_autoptr(RCUReadAuto) _rcu_read_auto __attribute__((unused)) =3D rcu_= read_auto_lock() =20 +/* + * Force-RCU notifiers tell readers that they should exit their + * read-side critical section. + */ +void rcu_add_force_rcu_notifier(Notifier *n, void *data); +void rcu_remove_force_rcu_notifier(Notifier *n); + #ifdef __cplusplus } #endif diff --git a/util/rcu.c b/util/rcu.c index 13ac0f75cb2a..0c68f068e23d 100644 --- a/util/rcu.c +++ b/util/rcu.c @@ -46,6 +46,7 @@ unsigned long rcu_gp_ctr =3D RCU_GP_LOCKED; =20 QemuEvent rcu_gp_event; +static int in_drain_call_rcu; static QemuMutex rcu_registry_lock; static QemuMutex rcu_sync_lock; =20 @@ -107,6 +108,8 @@ static void wait_for_readers(void) * get some extra futex wakeups. */ qatomic_set(&index->waiting, false); + } else if (qatomic_read(&in_drain_call_rcu)) { + notifier_list_notify(&index->force_rcu, index->force_rcu_d= ata); } } =20 @@ -293,7 +296,6 @@ void call_rcu1(struct rcu_head *node, void (*func)(stru= ct rcu_head *node)) qemu_event_set(&rcu_call_ready_event); } =20 - struct rcu_drain { struct rcu_head rcu; QemuEvent drain_complete_event; @@ -339,8 +341,10 @@ void drain_call_rcu(void) * assumed. */ =20 + qatomic_inc(&in_drain_call_rcu); call_rcu1(&rcu_drain.rcu, drain_rcu_callback); qemu_event_wait(&rcu_drain.drain_complete_event); + qatomic_dec(&in_drain_call_rcu); =20 if (locked) { qemu_mutex_lock_iothread(); @@ -363,6 +367,22 @@ void rcu_unregister_thread(void) qemu_mutex_unlock(&rcu_registry_lock); } =20 +void rcu_add_force_rcu_notifier(Notifier *n, void *data) +{ + qemu_mutex_lock(&rcu_registry_lock); + notifier_list_add(&rcu_reader.force_rcu, n); + rcu_reader.force_rcu_data =3D data; + qemu_mutex_unlock(&rcu_registry_lock); +} + +void rcu_remove_force_rcu_notifier(Notifier *n) +{ + qemu_mutex_lock(&rcu_registry_lock); + rcu_reader.force_rcu_data =3D NULL; + notifier_remove(n); + qemu_mutex_unlock(&rcu_registry_lock); +} + static void rcu_init_complete(void) { QemuThread thread; --=20 2.31.1 From nobody Tue May 21 23:09:31 2024 Delivered-To: importer@patchew.org Authentication-Results: mx.zohomail.com; spf=pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org Return-Path: Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) by mx.zohomail.com with SMTPS id 1634623082025906.4370215520764; Mon, 18 Oct 2021 22:58:02 -0700 (PDT) Received: from localhost ([::1]:53494 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mci8O-000107-Se for importer@patchew.org; Tue, 19 Oct 2021 01:58:00 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:37704) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mci7D-0007PK-6u for qemu-devel@nongnu.org; Tue, 19 Oct 2021 01:56:48 -0400 Received: from us-smtp-delivery-44.mimecast.com ([205.139.111.44]:49481) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mci7B-000326-Ey for qemu-devel@nongnu.org; Tue, 19 Oct 2021 01:56:46 -0400 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-526-RNYQXNBLP2Cp1CewDDXbdw-1; Tue, 19 Oct 2021 01:56:39 -0400 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 8C8CD108088C; Tue, 19 Oct 2021 05:56:38 +0000 (UTC) Received: from bahia.redhat.com (unknown [10.39.192.173]) by smtp.corp.redhat.com (Postfix) with ESMTP id 1A07E19C79; Tue, 19 Oct 2021 05:56:36 +0000 (UTC) X-MC-Unique: RNYQXNBLP2Cp1CewDDXbdw-1 From: Greg Kurz To: qemu-devel@nongnu.org Subject: [PATCH v2 2/2] accel/tcg: Register a force_rcu notifier Date: Tue, 19 Oct 2021 07:56:32 +0200 Message-Id: <20211019055632.252879-3-groug@kaod.org> In-Reply-To: <20211019055632.252879-1-groug@kaod.org> References: <20211019055632.252879-1-groug@kaod.org> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 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 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: softfail client-ip=205.139.111.44; 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: 2.1.23 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+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZM-MESSAGEID: 1634623082916100002 Content-Type: text/plain; charset="utf-8" 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, both in MTTCG and RR modes: int cpu_exec(CPUState *cpu) { [...] rcu_read_lock(); [...] while (!cpu_handle_exception(cpu, &ret)) { // Busy loop keeps vCPU here } [...] rcu_read_unlock(); return ret; } Have all vCPUs 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. The notifier implementation is shared by MTTCG and RR since both are affected. Suggested-by: Paolo Bonzini 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 | 3 +++ accel/tcg/tcg-accel-ops-rr.c | 3 +++ accel/tcg/tcg-accel-ops.c | 15 +++++++++++++++ accel/tcg/tcg-accel-ops.h | 2 ++ 4 files changed, 23 insertions(+) diff --git a/accel/tcg/tcg-accel-ops-mttcg.c b/accel/tcg/tcg-accel-ops-mttc= g.c index 847d2079d21f..ea4a3217ce3f 100644 --- a/accel/tcg/tcg-accel-ops-mttcg.c +++ b/accel/tcg/tcg-accel-ops-mttcg.c @@ -44,11 +44,13 @@ static void *mttcg_cpu_thread_fn(void *arg) { CPUState *cpu =3D arg; + Notifier force_rcu =3D { .notify =3D tcg_cpus_force_rcu }; =20 assert(tcg_enabled()); g_assert(!icount_enabled()); =20 rcu_register_thread(); + rcu_add_force_rcu_notifier(&force_rcu, cpu); tcg_register_thread(); =20 qemu_mutex_lock_iothread(); @@ -100,6 +102,7 @@ static void *mttcg_cpu_thread_fn(void *arg) =20 tcg_cpus_destroy(cpu); qemu_mutex_unlock_iothread(); + rcu_remove_force_rcu_notifier(&force_rcu); 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..fc0c4905caf5 100644 --- a/accel/tcg/tcg-accel-ops-rr.c +++ b/accel/tcg/tcg-accel-ops-rr.c @@ -144,9 +144,11 @@ static void rr_deal_with_unplugged_cpus(void) static void *rr_cpu_thread_fn(void *arg) { CPUState *cpu =3D arg; + Notifier force_rcu =3D { .notify =3D tcg_cpus_force_rcu }; =20 assert(tcg_enabled()); rcu_register_thread(); + rcu_add_force_rcu_notifier(&force_rcu, cpu); tcg_register_thread(); =20 qemu_mutex_lock_iothread(); @@ -255,6 +257,7 @@ 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; } diff --git a/accel/tcg/tcg-accel-ops.c b/accel/tcg/tcg-accel-ops.c index 1a8e8390bd60..7f0d2b06044a 100644 --- a/accel/tcg/tcg-accel-ops.c +++ b/accel/tcg/tcg-accel-ops.c @@ -91,6 +91,21 @@ void tcg_handle_interrupt(CPUState *cpu, int mask) } } =20 +static void do_nothing(CPUState *cpu, run_on_cpu_data d) +{ +} + +void tcg_cpus_force_rcu(Notifier *notify, void *data) +{ + CPUState *cpu =3D data; + + /* + * Called with rcu_registry_lock held, using async_run_on_cpu() ensures + * that there are no deadlocks. + */ + async_run_on_cpu(cpu, do_nothing, RUN_ON_CPU_NULL); +} + static void tcg_accel_ops_init(AccelOpsClass *ops) { if (qemu_tcg_mttcg_enabled()) { diff --git a/accel/tcg/tcg-accel-ops.h b/accel/tcg/tcg-accel-ops.h index 6a5fcef88980..8742041c8aea 100644 --- a/accel/tcg/tcg-accel-ops.h +++ b/accel/tcg/tcg-accel-ops.h @@ -18,5 +18,7 @@ void tcg_cpus_destroy(CPUState *cpu); int tcg_cpus_exec(CPUState *cpu); void tcg_handle_interrupt(CPUState *cpu, int mask); void tcg_cpu_init_cflags(CPUState *cpu, bool parallel); +/* Common force_rcu notifier for MTTCG and RR */ +void tcg_cpus_force_rcu(Notifier *notify, void *data); =20 #endif /* TCG_CPUS_H */ --=20 2.31.1