Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
Message-ID: <fa40ea62-5df8-4e18-af57-9ccd5c578e97@default>
Date:   Mon, 14 Jan 2019 01:25:41 -0800 (PST)
From:   Zhenzhong Duan <zhenzhong.duan@oracle.com>
To:     <longman@redhat.com>
Cc:     <srinivas.eeda@oracle.com>, <linux-kernel@vger.kernel.org>,
        <peterz@infradead.org>
Subject: Re: Question about qspinlock nest
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
Content-Disposition: inline
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


----- longman@redhat.com wrote:

> On 01/11/2019 12:06 AM, Zhenzhong Duan wrote:
> >
> >
> > On 2019/1/10 22:43, Waiman Long wrote:
> >> On 01/10/2019 03:02 AM, Zhenzhong Duan wrote:
> >>> Hi Maintainer,
> >>>
> >>>
> >>> There is a question confused me for days. Appreciate an answer.
> >>>
> >>> In below code, the comment says we never have more than 4 nested
> >>> contexts.
> >>>
> >>> What happen if debug and mce exceptions nest with the four, or we
> >>> ensure it never happen?
> >>>
> >>>
> >>> /*
> >>> =C2=A0* Per-CPU queue node structures; we can never have more than 4
> nested
> >>> =C2=A0* contexts: task, softirq, hardirq, nmi.
> >>> =C2=A0*
> >>> =C2=A0* Exactly fits one 64-byte cacheline on a 64-bit architecture.
> >>> =C2=A0*
> >>> =C2=A0* PV doubles the storage and uses the second cacheline for PV
> state.
> >>> =C2=A0*/
> >>> static DEFINE_PER_CPU_ALIGNED(struct qnode, qnodes[MAX_NODES]);
> >>>
> >> Yes, both debug and mce exceptions are some kind of NMIs. So
> >> theoretically, it is possible to have more than four. Are you aware
> of
> >> any debug and MCE exception handlers that need to take a spinlock
> for
> >> synchronization?
> > Not for debug exception, for MCE exception handler I found below
> two:
> >
> > do_machine_check->mce_report_event->schedule_work
> > do_machine_check->force_sig->force_sig_info
> >
> > schedule_work() and force_sig_info() take spinlocks.
> > --=20
> > Thanks
> > Zhenzhong
>=20
> The comment for do_machine_scheck() has already state that:
>=20
> =C2=A0* This is executed in NMI context not subject to normal locking
> rules. This
> =C2=A0* implies that most kernel services cannot be safely used. Don't
> even
> =C2=A0* think about putting a printk in there!
>=20
> So even if it doesn't exceed the MAX_NODES limit, it could hit
> deadlock
> and other kind of locking hazards. So supporting MCE is not a reason
> strong enough to extend MAX_NODES.

Agree.

>=20
> In hindsight, we should have added a "BUG_ON(idx >=3D MAX_NODES);"
> check
> to avoid silent corruption because of that issue.

Looks a good idea if it's hard to avoid using spinlock in MCE handler.

Thanks
Zhenzhong