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Message-ID: <1922911f0b27bf60a64e8c1fa56f2247585dd481.camel@infradead.org>
Subject: Re: [PATCH v10 17/19] KVM: xen: don't block on pfncache locks in
 kvm_xen_set_evtchn_fast()
From:   David Woodhouse <dwmw2@infradead.org>
To:     Paul Durrant <paul@xen.org>, Paolo Bonzini <pbonzini@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Sean Christopherson <seanjc@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, Shuah Khan <shuah@kernel.org>,
        kvm@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org
Date:   Thu, 14 Dec 2023 14:03:38 +0000
In-Reply-To: <20231204144334.910-18-paul@xen.org>
References: <20231204144334.910-1-paul@xen.org>
         <20231204144334.910-18-paul@xen.org>
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On Mon, 2023-12-04 at 14:43 +0000, Paul Durrant wrote:
> From: Paul Durrant <pdurrant@amazon.com>
>=20
> As described in [1] compiling with CONFIG_PROVE_RAW_LOCK_NESTING shows th=
at
> 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.
>=20
> [1] https://lore.kernel.org/lkml/99771ef3a4966a01fefd3adbb2ba9c3a75f97cf2=
.camel@infradead.org/T/#mbd06e5a04534ce9c0ee94bd8f1e8d942b2d45bd6
>=20
> Signed-off-by: Paul Durrant <pdurrant@amazon.com>

Reviewed-by: David Woodhouse <dwmw@amazon.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 =E2=80=94 and mostly trick th=
ings
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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