linux-kernel - Re: [PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary

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Message-ID: <0e30625f-934d-9084-e293-cb3bcbc9e4b8@redhat.com>
Date:   Wed, 31 Mar 2021 22:42:30 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>, Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH 16/18] KVM: Don't take mmu_lock for range invalidation
 unless necessary

On 31/03/21 21:47, Sean Christopherson wrote:
> Rereading things, a small chunk of the rwsem nastiness can go away.  I don't see
> any reason to use rw_semaphore instead of rwlock_t.

Wouldn't it be incorrect to lock a mutex (e.g. inside *another* MMU 
notifier's invalidate callback) while holding an rwlock_t?  That makes 
sense because anybody that's busy waiting in write_lock potentially 
cannot be preempted until the other task gets the mutex.  This is a 
potential deadlock.

I also thought of busy waiting on down_read_trylock if the MMU notifier 
cannot block, but that would also be invalid for the opposite reason 
(the down_write task might be asleep, waiting for other readers to 
release the task, and the down_read_trylock busy loop might not let that 
task run).

> And that's _already_ the worst case since notifications are currently
> serialized by mmu_lock.

But right now notifications are not a single critical section, they're 
two, aren't they?

Paolo