Re: [PATCH v12 17/20] KVM: xen: don't block on pfncache locks in kvm_xen_set_evtchn_fast()

From: David Woodhouse <dwmw2@infradead.org>
To: Sean Christopherson <seanjc@google.com>, Paul Durrant <paul@xen.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Jonathan Corbet <corbet@lwn.net>,
	 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
Subject: Re: [PATCH v12 17/20] KVM: xen: don't block on pfncache locks in kvm_xen_set_evtchn_fast()
Date: Tue, 06 Feb 2024 20:21:33 -0800	[thread overview]
Message-ID: <c7e17e35e800bb89e7691fb209a527fda2beb743.camel@infradead.org> (raw)
In-Reply-To: <ZcMEWMAKCDp5sA9r@google.com>

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On Tue, 2024-02-06 at 20:17 -0800, Sean Christopherson wrote:
> On Mon, Jan 15, 2024, Paul Durrant wrote:
> > From: Paul Durrant <pdurrant@amazon.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.

I don't know about "fundamentally incapable", but there's no point in
adding them just for this, because the write lock is very rarely going
to be held, so it's not an issue to be falling back to the slow path.
And when the write lock *was* held, something often changed so we may
well be going back to the slow path anyway.

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