linux-kernel - Re: [PATCH v10 17/19] KVM: xen: don't block on pfncache locks in kvm_xen_set_evtchn

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Date:   Thu, 14 Dec 2023 14:03:38 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Paul Durrant <paul@....org>, Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Shuah Khan <shuah@...nel.org>,
        kvm@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v10 17/19] KVM: xen: don't block on pfncache locks in
 kvm_xen_set_evtchn_fast()

On Mon, 2023-12-04 at 14:43 +0000, Paul Durrant wrote:
> From: Paul Durrant <pdurrant@...zon.com>
> 
> As described in [1] compiling with CONFIG_PROVE_RAW_LOCK_NESTING shows that
> kvm_xen_set_evtchn_fast() is blocking on pfncache locks in IRQ context.
> Instead, use read_trylock() and treat failure to lock the same as an
> invalid cache.
> 
> [1] https://lore.kernel.org/lkml/99771ef3a4966a01fefd3adbb2ba9c3a75f97cf2.camel@infradead.org/T/#mbd06e5a04534ce9c0ee94bd8f1e8d942b2d45bd6
> 
> Signed-off-by: Paul Durrant <pdurrant@...zon.com>

Reviewed-by: David Woodhouse <dwmw@...zon.co.uk>

Although I would prefer the commit comment to note that this is only
wrong for PREEMPT_RT, as all those locks were irq-safe. This is because
PREEMPT_RT is going to turn them into mutexes — and mostly trick things
into working by making all IRQ handlers threaded... except this one
isn't just in an IRQ handler; its in an HRTIMER_MODE_ABS_HARD callback
which still *will* be in real interrupt context.

And there's no 'raw' version of the rwlock for us to use.

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