From: Paolo Bonzini <pbonzini@redhat.com>
To: Sean Christopherson <seanjc@google.com>,
Oliver Upton <oupton@google.com>
Cc: kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu,
Marc Zyngier <maz@kernel.org>, Peter Shier <pshier@google.com>,
Jim Mattson <jmattson@google.com>,
David Matlack <dmatlack@google.com>,
Ricardo Koller <ricarkol@google.com>,
Jing Zhang <jingzhangos@google.com>,
Raghavendra Rao Anata <rananta@google.com>,
James Morse <james.morse@arm.com>,
Alexandru Elisei <alexandru.elisei@arm.com>,
Suzuki K Poulose <suzuki.poulose@arm.com>,
linux-arm-kernel@lists.infradead.org,
Andrew Jones <drjones@redhat.com>, Will Deacon <will@kernel.org>,
Catalin Marinas <catalin.marinas@arm.com>
Subject: Re: [PATCH v7 5/6] KVM: x86: Refactor tsc synchronization code
Date: Fri, 24 Sep 2021 11:28:28 +0200 [thread overview]
Message-ID: <de3ff2de-da79-fd0c-c90b-f767414b0b69@redhat.com> (raw)
In-Reply-To: <YTEkRfTFyoh+HQyT@google.com>
On 02/09/21 21:21, Sean Christopherson wrote:
>
>> + if (!matched) {
>> + ...
>> + spin_lock(&kvm->arch.pvclock_gtod_sync_lock);
>> + kvm->arch.nr_vcpus_matched_tsc = 0;
>> + } else if (!already_matched) {
>> + spin_lock(&kvm->arch.pvclock_gtod_sync_lock);
>> + kvm->arch.nr_vcpus_matched_tsc++;
>> + }
>> +
>> + kvm_track_tsc_matching(vcpu);
>> + spin_unlock(&kvm->arch.pvclock_gtod_sync_lock);
>
> This unlock is imbalanced if matched and already_matched are both true. It's not
> immediately obvious that that_can't_ happen, and if it truly can't happen then
> conditionally locking is pointless (because it's not actually conditional).
This is IMO another reason to unify tsc_write_lock and
pvclock_gtod_sync_lock. The chances of contention are pretty slim. As
soon as I sort out the next -rc3 pull request I'll send out my version
of Oliver's patches.
Thanks,
Paolo
next prev parent reply other threads:[~2021-09-24 9:28 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-16 0:11 [PATCH v7 0/6] KVM: x86: Add idempotent controls for migrating system counter state Oliver Upton
2021-08-16 0:11 ` [PATCH v7 1/6] KVM: x86: Fix potential race in KVM_GET_CLOCK Oliver Upton
2021-08-19 18:24 ` Marcelo Tosatti
2021-08-20 18:22 ` Oliver Upton
2021-08-16 0:11 ` [PATCH v7 2/6] KVM: x86: Create helper methods for KVM_{GET,SET}_CLOCK ioctls Oliver Upton
2021-08-16 0:11 ` [PATCH v7 3/6] KVM: x86: Report host tsc and realtime values in KVM_GET_CLOCK Oliver Upton
2021-08-20 12:46 ` Marcelo Tosatti
2021-09-24 8:30 ` Paolo Bonzini
2021-08-16 0:11 ` [PATCH v7 4/6] KVM: x86: Take the pvclock sync lock behind the tsc_write_lock Oliver Upton
2021-09-02 19:22 ` Sean Christopherson
2021-08-16 0:11 ` [PATCH v7 5/6] KVM: x86: Refactor tsc synchronization code Oliver Upton
2021-09-02 19:21 ` Sean Christopherson
2021-09-02 19:41 ` Oliver Upton
2021-09-24 9:28 ` Paolo Bonzini [this message]
2021-08-16 0:11 ` [PATCH v7 6/6] KVM: x86: Expose TSC offset controls to userspace Oliver Upton
2021-08-23 20:56 ` Oliver Upton
2021-08-26 12:48 ` Marcelo Tosatti
2021-08-26 20:27 ` Oliver Upton
2021-09-02 19:23 ` [PATCH v7 0/6] KVM: x86: Add idempotent controls for migrating system counter state Sean Christopherson
2021-09-02 19:45 ` Oliver Upton
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