linux-kernel - Re: [PATCH v12 17/20] KVM: xen: don't block on pfncache locks in kvm_xen_set_evtchn

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Message-ID: <ZcMEWMAKCDp5sA9r@google.com>
Date: Tue, 6 Feb 2024 20:17:28 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Paul Durrant <paul@....org>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>, 
	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>, David Woodhouse <dwmw2@...radead.org>, 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 v12 17/20] KVM: xen: don't block on pfncache locks in kvm_xen_set_evtchn_fast()

On Mon, Jan 15, 2024, 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.
> There is only actually blocking with PREEMPT_RT because the locks will
> turned into mutexes. There is no 'raw' version of rwlock_t that can be used
> to avoid that, so use read_trylock() and treat failure to lock the same as
> an invalid cache.

Are rwlocks fundamentally incapable of supporting a raw version?  Because that's
the only argument I see for adding a hack like this.