linux-kernel - Re: [Xen-devel] [PATCH 2/2] xen: make xen_qlock

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Message-Id: <5BB1E18802000078001ED127@prv1-mh.provo.novell.com>
Date:   Mon, 01 Oct 2018 02:57:44 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Juergen Gross" <jgross@...e.com>
Cc:     "Borislav Petkov" <bp@...en8.de>,
        "Peter Zijlstra" <peterz@...radead.org>,
        "the arch/x86 maintainers" <x86@...nel.org>, <tglx@...utronix.de>,
        "xen-devel" <xen-devel@...ts.xenproject.org>,
        "Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
        <longman@...hat.com>, <mingo@...hat.com>,
        <linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>,
        <hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH 2/2] xen: make xen_qlock_wait()
 nestable

>>> On 01.10.18 at 09:16, <jgross@...e.com> wrote:
> xen_qlock_wait() isn't safe for nested calls due to interrupts. A call
> of xen_qlock_kick() might be ignored in case a deeper nesting level
> was active right before the call of xen_poll_irq():
> 
> CPU 1:                                   CPU 2:
> spin_lock(lock1)
>                                          spin_lock(lock1)
>                                          -> xen_qlock_wait()
>                                             -> xen_clear_irq_pending()
>                                             Interrupt happens
> spin_unlock(lock1)
> -> xen_qlock_kick(CPU 2)
> spin_lock_irqsave(lock2)
>                                          spin_lock_irqsave(lock2)
>                                          -> xen_qlock_wait()
>                                             -> xen_clear_irq_pending()
>                                                clears kick for lock1
>                                             -> xen_poll_irq()
> spin_unlock_irq_restore(lock2)
> -> xen_qlock_kick(CPU 2)
>                                             wakes up
>                                          spin_unlock_irq_restore(lock2)
>                                          IRET
>                                            resumes in xen_qlock_wait()
>                                            -> xen_poll_irq()
>                                            never wakes up
> 
> The solution is to disable interrupts in xen_qlock_wait() and not to
> poll for the irq in case xen_qlock_wait() is called in nmi context.

Are precautions against NMI really worthwhile? Locks acquired both
in NMI context as well as outside of it are liable to deadlock anyway,
aren't they?

Jan