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[60.242.147.73]) by smtp.gmail.com with ESMTPSA id f28sm694974pgb.12.2021.06.15.21.28.58 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 15 Jun 2021 21:28:59 -0700 (PDT) Date: Wed, 16 Jun 2021 14:28:54 +1000 From: Nicholas Piggin Subject: Re: [PATCH 5/8] membarrier, kthread: Use _ONCE accessors for task->mm To: Andy Lutomirski , x86@kernel.org Cc: Andrew Morton , Dave Hansen , LKML , linux-mm@kvack.org, Mathieu Desnoyers , Peter Zijlstra References: <74ace142f48db7d0e71b05b5ace72bfe8e0a2652.1623813516.git.luto@kernel.org> In-Reply-To: <74ace142f48db7d0e71b05b5ace72bfe8e0a2652.1623813516.git.luto@kernel.org> MIME-Version: 1.0 Message-Id: <1623817648.jh19a97b5a.astroid@bobo.none> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Authentication-Results: imf01.hostedemail.com; dkim=pass header.d=gmail.com header.s=20161025 header.b=LuXEXas8; dmarc=pass (policy=none) header.from=gmail.com; spf=pass (imf01.hostedemail.com: domain of npiggin@gmail.com designates 209.85.215.175 as permitted sender) smtp.mailfrom=npiggin@gmail.com X-Stat-Signature: e4zk9xwa3jya5r1jpka7d85uy5iu7m5i X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 6804F5001703 X-HE-Tag: 1623817730-403564 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Excerpts from Andy Lutomirski's message of June 16, 2021 1:21 pm: > membarrier reads cpu_rq(remote cpu)->curr->mm without locking. Use > READ_ONCE() and WRITE_ONCE() to remove the data races. >=20 > Cc: Mathieu Desnoyers > Cc: Nicholas Piggin > Cc: Peter Zijlstra > Signed-off-by: Andy Lutomirski > --- > fs/exec.c | 2 +- > kernel/kthread.c | 4 ++-- > kernel/sched/membarrier.c | 6 +++--- > 3 files changed, 6 insertions(+), 6 deletions(-) >=20 > diff --git a/fs/exec.c b/fs/exec.c > index 18594f11c31f..2e63dea83411 100644 > --- a/fs/exec.c > +++ b/fs/exec.c > @@ -1007,7 +1007,7 @@ static int exec_mmap(struct mm_struct *mm) > local_irq_disable(); > active_mm =3D tsk->active_mm; > tsk->active_mm =3D mm; > - tsk->mm =3D mm; > + WRITE_ONCE(tsk->mm, mm); /* membarrier reads this without locks */ > /* > * This prevents preemption while active_mm is being loaded and > * it and mm are being updated, which could cause problems for > diff --git a/kernel/kthread.c b/kernel/kthread.c > index 8275b415acec..4962794e02d5 100644 > --- a/kernel/kthread.c > +++ b/kernel/kthread.c > @@ -1322,7 +1322,7 @@ void kthread_use_mm(struct mm_struct *mm) > mmgrab(mm); > tsk->active_mm =3D mm; > } > - tsk->mm =3D mm; > + WRITE_ONCE(tsk->mm, mm); /* membarrier reads this without locks */ > membarrier_update_current_mm(mm); > switch_mm_irqs_off(active_mm, mm, tsk); > membarrier_finish_switch_mm(atomic_read(&mm->membarrier_state)); > @@ -1363,7 +1363,7 @@ void kthread_unuse_mm(struct mm_struct *mm) > smp_mb__after_spinlock(); > sync_mm_rss(mm); > local_irq_disable(); > - tsk->mm =3D NULL; > + WRITE_ONCE(tsk->mm, NULL); /* membarrier reads this without locks */ > membarrier_update_current_mm(NULL); > /* active_mm is still 'mm' */ > enter_lazy_tlb(mm, tsk); > diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c > index 3173b063d358..c32c32a2441e 100644 > --- a/kernel/sched/membarrier.c > +++ b/kernel/sched/membarrier.c > @@ -410,7 +410,7 @@ static int membarrier_private_expedited(int flags, in= t cpu_id) > goto out; > rcu_read_lock(); > p =3D rcu_dereference(cpu_rq(cpu_id)->curr); > - if (!p || p->mm !=3D mm) { > + if (!p || READ_ONCE(p->mm) !=3D mm) { > rcu_read_unlock(); > goto out; > } > @@ -423,7 +423,7 @@ static int membarrier_private_expedited(int flags, in= t cpu_id) > struct task_struct *p; > =20 > p =3D rcu_dereference(cpu_rq(cpu)->curr); > - if (p && p->mm =3D=3D mm) > + if (p && READ_ONCE(p->mm) =3D=3D mm) /* exec, kthread_use_mm write this without locks */ ? Seems good to me. Acked-by: Nicholas Piggin > __cpumask_set_cpu(cpu, tmpmask); > } > rcu_read_unlock(); > @@ -521,7 +521,7 @@ static int sync_runqueues_membarrier_state(struct mm_= struct *mm) > struct task_struct *p; > =20 > p =3D rcu_dereference(rq->curr); > - if (p && p->mm =3D=3D mm) > + if (p && READ_ONCE(p->mm) =3D=3D mm) > __cpumask_set_cpu(cpu, tmpmask); > } > rcu_read_unlock(); > --=20 > 2.31.1 >=20 >=20