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Wed, 10 May 2023 09:03:48 -0700 (PDT) X-Google-Smtp-Source: ACHHUZ5ejKJ2nhJTnUCYc8+PzokcqNo9w5tzeRdRn9+d7zzaZPyTHEPJ7u1gi2RnLsTWKywSP+AXDQ== X-Received: by 2002:a17:906:fe02:b0:94b:4a4:2836 with SMTP id wy2-20020a170906fe0200b0094b04a42836mr14847211ejb.69.1683734627736; Wed, 10 May 2023 09:03:47 -0700 (PDT) From: Paolo Bonzini To: qemu-devel@nongnu.org Cc: richard.henderson@linaro.org Subject: [PATCH] tcg: round-robin: do not use mb_read for rr_current_cpu Date: Wed, 10 May 2023 18:03:45 +0200 Message-Id: <20230510160346.1248626-1-pbonzini@redhat.com> X-Mailer: git-send-email 2.40.1 MIME-Version: 1.0 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: pass client-ip=170.10.129.124; envelope-from=pbonzini@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham 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: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1683734676810100002 Content-Type: text/plain; charset="utf-8" Note that qatomic_mb_set can remain, similar to how Linux has smp_store_mb (an optimized version of following a store with a full memory barrier). Signed-off-by: Paolo Bonzini --- accel/tcg/tcg-accel-ops-rr.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) diff --git a/accel/tcg/tcg-accel-ops-rr.c b/accel/tcg/tcg-accel-ops-rr.c index 290833a37fb2..055f6ae29553 100644 --- a/accel/tcg/tcg-accel-ops-rr.c +++ b/accel/tcg/tcg-accel-ops-rr.c @@ -71,11 +71,13 @@ static void rr_kick_next_cpu(void) { CPUState *cpu; do { - cpu =3D qatomic_mb_read(&rr_current_cpu); + cpu =3D qatomic_read(&rr_current_cpu); if (cpu) { cpu_exit(cpu); } - } while (cpu !=3D qatomic_mb_read(&rr_current_cpu)); + /* Finish kicking this cpu before reading again. */ + smp_mb(); + } while (cpu !=3D qatomic_read(&rr_current_cpu)); } =20 static void rr_kick_thread(void *opaque) @@ -206,8 +208,9 @@ static void *rr_cpu_thread_fn(void *arg) } =20 while (cpu && cpu_work_list_empty(cpu) && !cpu->exit_request) { - + /* Store rr_current_cpu before evaluating cpu_can_run(). */ qatomic_mb_set(&rr_current_cpu, cpu); + current_cpu =3D cpu; =20 qemu_clock_enable(QEMU_CLOCK_VIRTUAL, @@ -245,7 +248,7 @@ static void *rr_cpu_thread_fn(void *arg) cpu =3D CPU_NEXT(cpu); } /* while (cpu && !cpu->exit_request).. */ =20 - /* Does not need qatomic_mb_set because a spurious wakeup is okay.= */ + /* Does not need a memory barrier because a spurious wakeup is oka= y. */ qatomic_set(&rr_current_cpu, NULL); =20 if (cpu && cpu->exit_request) { --=20 2.40.1