From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751464AbdFFMyG (ORCPT ); Tue, 6 Jun 2017 08:54:06 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:58145 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751305AbdFFMyF (ORCPT ); Tue, 6 Jun 2017 08:54:05 -0400 Date: Tue, 6 Jun 2017 05:53:57 -0700 From: "Paul E. McKenney" To: Paolo Bonzini Cc: Peter Zijlstra , linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, kvm@vger.kernel.org, Linus Torvalds Subject: Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context Reply-To: paulmck@linux.vnet.ibm.com References: <20170605220919.GA27820@linux.vnet.ibm.com> <1496700591-30177-1-git-send-email-paulmck@linux.vnet.ibm.com> <20170606110915.g7zlzoj5iidhpp7g@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17060612-0056-0000-0000-0000037EE46B X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007183; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000212; SDB=6.00870941; UDB=6.00433128; IPR=6.00650933; BA=6.00005401; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00015718; XFM=3.00000015; UTC=2017-06-06 12:54:02 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17060612-0057-0000-0000-000007B4EA81 Message-Id: <20170606125357.GB3721@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-06-06_10:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1703280000 definitions=main-1706060215 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 06, 2017 at 02:01:54PM +0200, Paolo Bonzini wrote: > > > On 06/06/2017 13:09, Peter Zijlstra wrote: > >> diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c > >> index 36e1f82faed1..681bf6bc04a5 100644 > >> --- a/kernel/rcu/srcutiny.c > >> +++ b/kernel/rcu/srcutiny.c > >> @@ -35,8 +35,8 @@ > >> > >> static int init_srcu_struct_fields(struct srcu_struct *sp) > >> { > >> - sp->srcu_lock_nesting[0] = 0; > >> - sp->srcu_lock_nesting[1] = 0; > >> + atomic_set(&sp->srcu_lock_nesting[0], 0); > >> + atomic_set(&sp->srcu_lock_nesting[1], 0); > >> init_swait_queue_head(&sp->srcu_wq); > >> sp->srcu_gp_seq = 0; > >> rcu_segcblist_init(&sp->srcu_cblist); > >> @@ -86,7 +86,8 @@ EXPORT_SYMBOL_GPL(init_srcu_struct); > >> */ > >> void cleanup_srcu_struct(struct srcu_struct *sp) > >> { > >> - WARN_ON(sp->srcu_lock_nesting[0] || sp->srcu_lock_nesting[1]); > >> + WARN_ON(atomic_read(&sp->srcu_lock_nesting[0]) || > >> + atomic_read(&sp->srcu_lock_nesting[1])); > >> flush_work(&sp->srcu_work); > >> WARN_ON(rcu_seq_state(sp->srcu_gp_seq)); > >> WARN_ON(sp->srcu_gp_running); > >> @@ -97,7 +98,7 @@ 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) > >> @@ -105,21 +106,19 @@ int __srcu_read_lock(struct srcu_struct *sp) > >> int idx; > >> > >> idx = READ_ONCE(sp->srcu_idx); > >> - WRITE_ONCE(sp->srcu_lock_nesting[idx], sp->srcu_lock_nesting[idx] + 1); > >> + atomic_inc(&sp->srcu_lock_nesting[idx]); > >> return idx; > >> } > >> 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) > >> { > >> - int newval = sp->srcu_lock_nesting[idx] - 1; > >> - > >> - WRITE_ONCE(sp->srcu_lock_nesting[idx], newval); > >> - if (!newval && READ_ONCE(sp->srcu_gp_waiting)) > >> + if (atomic_dec_return_relaxed(&sp->srcu_lock_nesting[idx]) == 0 && > >> + READ_ONCE(sp->srcu_gp_waiting)) > >> swake_up(&sp->srcu_wq); > >> } > >> EXPORT_SYMBOL_GPL(__srcu_read_unlock); > >> @@ -148,7 +147,7 @@ void srcu_drive_gp(struct work_struct *wp) > >> idx = sp->srcu_idx; > >> WRITE_ONCE(sp->srcu_idx, !sp->srcu_idx); > >> WRITE_ONCE(sp->srcu_gp_waiting, true); /* srcu_read_unlock() wakes! */ > >> - swait_event(sp->srcu_wq, !READ_ONCE(sp->srcu_lock_nesting[idx])); > >> + swait_event(sp->srcu_wq, !atomic_read(&sp->srcu_lock_nesting[idx])); > >> WRITE_ONCE(sp->srcu_gp_waiting, false); /* srcu_read_unlock() cheap. */ > >> rcu_seq_end(&sp->srcu_gp_seq); > > > > I'm not entirely sure this is actually needed. TINY_SRCU is !PREEMPT && > > !SMP. So that means all we need is to be safe from IRQs. > > > > Now, do we (want) support things like: > > > > > > srcu_read_lock(); > > > > > > srcu_read_lock(); > > > > srcu_read_unlock(); > > > > > > srcu_read_unlock(); > > > > > > > > _OR_ > > > > do we already (or want to) mandate that SRCU usage in IRQs must be > > balanced? That is, if it is used from IRQ context it must do an equal > > amount of srcu_read_lock() and srcu_read_unlock()s? > > > > Because if we have the balance requirement (as we do for > > preempt_disable()) then even on load-store architectures the current > > code should be sufficient (since if an interrupt does as many dec's as > > it does inc's, the actual value will not change over an interrupt, and > > our load from before the interrupt is still valid). > > Good point! So the srcutiny part should not be necessary. I'll reply > to the other email now. Good analysis, Peter! So the only part of this patch that is needed is the changes to the comments, right? ;-) Thanx, Paul