From patchwork Tue Jun 6 17:07:26 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Paul E. McKenney" X-Patchwork-Id: 9769303 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id 96A136035D for ; Tue, 6 Jun 2017 17:08:13 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 8F9BA2013D for ; Tue, 6 Jun 2017 17:08:13 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 82E7B2621B; Tue, 6 Jun 2017 17:08:13 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=2.0 tests=BAYES_00,RCVD_IN_DNSWL_HI autolearn=unavailable version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 721792013D for ; Tue, 6 Jun 2017 17:08:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751606AbdFFRHg (ORCPT ); Tue, 6 Jun 2017 13:07:36 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:40325 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751448AbdFFRHf (ORCPT ); Tue, 6 Jun 2017 13:07:35 -0400 Received: from pps.filterd (m0098414.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.20/8.16.0.20) with SMTP id v56H4D67065605 for ; Tue, 6 Jun 2017 13:07:34 -0400 Received: from e13.ny.us.ibm.com (e13.ny.us.ibm.com [129.33.205.203]) by mx0b-001b2d01.pphosted.com with ESMTP id 2awnkn39qy-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Tue, 06 Jun 2017 13:07:34 -0400 Received: from localhost by e13.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 6 Jun 2017 13:07:33 -0400 Received: from b01cxnp22035.gho.pok.ibm.com (9.57.198.25) by e13.ny.us.ibm.com (146.89.104.200) with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted; Tue, 6 Jun 2017 13:07:28 -0400 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22035.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id v56H7QF264815236; Tue, 6 Jun 2017 17:07:28 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id D668BB2052; Tue, 6 Jun 2017 13:05:03 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.110]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP id 957BCB2050; Tue, 6 Jun 2017 13:05:03 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id EB3D216C6124; Tue, 6 Jun 2017 10:07:28 -0700 (PDT) From: "Paul E. McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, Paolo Bonzini , kvm@vger.kernel.org, Linus Torvalds , "Paul E. McKenney" Subject: [PATCH v2 tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context Date: Tue, 6 Jun 2017 10:07:26 -0700 X-Mailer: git-send-email 2.5.2 In-Reply-To: <@@@> References: <@@@> X-TM-AS-GCONF: 00 x-cbid: 17060617-0008-0000-0000-000002380493 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007184; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000212; SDB=6.00871026; UDB=6.00433179; IPR=6.00651018; BA=6.00005403; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00015720; XFM=3.00000015; UTC=2017-06-06 17:07:32 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17060617-0009-0000-0000-0000358F164A Message-Id: <1496768847-13732-1-git-send-email-paulmck@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:, , definitions=2017-06-06_12:, , signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=3 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1703280000 definitions=main-1706060294 Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP From: Paolo Bonzini Linu Cherian reported a WARN in cleanup_srcu_struct() when shutting down a guest running iperf on a VFIO assigned device. This happens because irqfd_wakeup() calls srcu_read_lock(&kvm->irq_srcu) in interrupt context, while a worker thread does the same inside kvm_set_irq(). If the interrupt happens while the worker thread is executing __srcu_read_lock(), updates to the Classic SRCU ->lock_count[] field or the Tree SRCU ->srcu_lock_count[] field can be lost. The docs say you are not supposed to call srcu_read_lock() and srcu_read_unlock() from irq context, but KVM interrupt injection happens from (host) interrupt context and it would be nice if SRCU supported the use case. KVM is using SRCU here not really for the "sleepable" part, but rather due to its IPI-free fast detection of grace periods. It is therefore not desirable to switch back to RCU, which would effectively revert commit 719d93cd5f5c ("kvm/irqchip: Speed up KVM_SET_GSI_ROUTING", 2014-01-16). However, the docs are overly conservative. You can have an SRCU instance only has users in irq context, and you can mix process and irq context as long as process context users disable interrupts. In addition, __srcu_read_unlock() actually uses this_cpu_dec() on both Tree SRCU and Classic SRCU. For those two implementations, only srcu_read_lock() is unsafe. When Classic SRCU's __srcu_read_unlock() was changed to use this_cpu_dec(), in commit 5a41344a3d83 ("srcu: Simplify __srcu_read_unlock() via this_cpu_dec()", 2012-11-29), __srcu_read_lock() did two increments. Therefore it kept __this_cpu_inc(), with preempt_disable/enable in the caller. Tree SRCU however only does one increment, so on most architectures it is more efficient for __srcu_read_lock() to use this_cpu_inc(), and any performance differences appear to be down in the noise. Unlike Classic and Tree SRCU, Tiny SRCU does increments and decrements on a single variable. Therefore, as Peter Zijlstra pointed out, Tiny SRCU's implementation already supports mixed-context use of srcu_read_lock() and srcu_read_unlock(), at least as long as uses of srcu_read_lock() and srcu_read_unlock() in each handler are nested and paired properly. In other words, it is still illegal to (say) invoke srcu_read_lock() in an interrupt handler and to invoke the matching srcu_read_unlock() in a softirq handler. Therefore, the only change required for Tiny SRCU is to its comments. Fixes: 719d93cd5f5c ("kvm/irqchip: Speed up KVM_SET_GSI_ROUTING") Reported-by: Linu Cherian Suggested-by: Linu Cherian Cc: kvm@vger.kernel.org Signed-off-by: Paolo Bonzini Cc: Linus Torvalds Signed-off-by: Paul E. McKenney --- kernel/rcu/srcutiny.c | 7 ++++--- kernel/rcu/srcutree.c | 5 ++--- 2 files changed, 6 insertions(+), 6 deletions(-) diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c index 36e1f82faed1..32798eb14853 100644 --- a/kernel/rcu/srcutiny.c +++ b/kernel/rcu/srcutiny.c @@ -97,8 +97,9 @@ EXPORT_SYMBOL_GPL(cleanup_srcu_struct); /* * Counts the new reader in the appropriate per-CPU element of the - * srcu_struct. Must be called from process context. - * Returns an index that must be passed to the matching srcu_read_unlock(). + * srcu_struct. Can be invoked from irq/bh handlers, but the matching + * __srcu_read_unlock() must be in the same handler instance. Returns an + * index that must be passed to the matching srcu_read_unlock(). */ int __srcu_read_lock(struct srcu_struct *sp) { @@ -112,7 +113,7 @@ EXPORT_SYMBOL_GPL(__srcu_read_lock); /* * Removes the count for the old reader from the appropriate element of - * the srcu_struct. Must be called from process context. + * the srcu_struct. */ void __srcu_read_unlock(struct srcu_struct *sp, int idx) { diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index 3ae8474557df..157654fa436a 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -357,7 +357,7 @@ EXPORT_SYMBOL_GPL(cleanup_srcu_struct); /* * Counts the new reader in the appropriate per-CPU element of the - * srcu_struct. Must be called from process context. + * srcu_struct. * Returns an index that must be passed to the matching srcu_read_unlock(). */ int __srcu_read_lock(struct srcu_struct *sp) @@ -365,7 +365,7 @@ int __srcu_read_lock(struct srcu_struct *sp) int idx; idx = READ_ONCE(sp->srcu_idx) & 0x1; - __this_cpu_inc(sp->sda->srcu_lock_count[idx]); + this_cpu_inc(sp->sda->srcu_lock_count[idx]); smp_mb(); /* B */ /* Avoid leaking the critical section. */ return idx; } @@ -375,7 +375,6 @@ EXPORT_SYMBOL_GPL(__srcu_read_lock); * Removes the count for the old reader from the appropriate per-CPU * element of the srcu_struct. Note that this may well be a different * CPU than that which was incremented by the corresponding srcu_read_lock(). - * Must be called from process context. */ void __srcu_read_unlock(struct srcu_struct *sp, int idx) {