From: Paolo Bonzini <pbonzini@redhat.com>
To: Peter Gonda <pgonda@google.com>
Cc: John Sperbeck <jsperbeck@google.com>,
kvm list <kvm@vger.kernel.org>,
David Rientjes <rientjes@google.com>,
Sean Christopherson <seanjc@google.com>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v3] KVM: SEV: Mark nested locking of vcpu->lock
Date: Fri, 29 Apr 2022 01:59:15 +0200 [thread overview]
Message-ID: <0d282be4-d612-374d-84ba-067994321bab@redhat.com> (raw)
In-Reply-To: <CAMkAt6rmDrZfN5DbNOTsKFV57PwEnK2zxgBTCbEPeE206+5v5w@mail.gmail.com>
On 4/28/22 23:28, Peter Gonda wrote:
>
> So when actually trying this out I noticed that we are releasing the
> current vcpu iterator but really we haven't actually taken that lock
> yet. So we'd need to maintain a prev_* pointer and release that one.
Not entirely true because all vcpu->mutex.dep_maps will be for the same
lock. The dep_map is essentially a fancy string, in this case
"&vcpu->mutex".
See the definition of mutex_init:
#define mutex_init(mutex) \
do { \
static struct lock_class_key __key; \
\
__mutex_init((mutex), #mutex, &__key); \
} while (0)
and the dep_map field is initialized with
lockdep_init_map_wait(&lock->dep_map, name, key, 0, LD_WAIT_SLEEP);
(i.e. all vcpu->mutexes share the same name and key because they have a
single mutex_init-ialization site). Lockdep is as crude in theory as it
is effective in practice!
>
> bool acquired = false;
> kvm_for_each_vcpu(...) {
> if (!acquired) {
> if (mutex_lock_killable_nested(&vcpu->mutex, role)
> goto out_unlock;
> acquired = true;
> } else {
> if (mutex_lock_killable(&vcpu->mutex, role)
> goto out_unlock;
This will cause a lockdep splat because it uses subclass 0. All the
*_nested functions is allow you to specify a subclass other than zero.
Paolo
> }
> }
>
> To unlock:
>
> kvm_for_each_vcpu(...) {
> mutex_unlock(&vcpu->mutex);
> }
>
> This way instead of mocking and releasing the lock_dep we just lock
> the fist vcpu with mutex_lock_killable_nested(). I think this
> maintains the property you suggested of "coalesces all the mutexes for
> a vm in a single subclass". Thoughts?
next prev parent reply other threads:[~2022-04-28 23:59 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-04-07 19:59 [PATCH v3] KVM: SEV: Mark nested locking of vcpu->lock Peter Gonda
2022-04-07 21:17 ` John Sperbeck
2022-04-08 15:08 ` Peter Gonda
2022-04-20 20:14 ` Peter Gonda
2022-04-21 15:56 ` Paolo Bonzini
2022-04-26 19:06 ` Peter Gonda
2022-04-27 16:04 ` Paolo Bonzini
2022-04-27 20:18 ` Peter Gonda
2022-04-28 21:28 ` Peter Gonda
2022-04-28 23:59 ` Paolo Bonzini [this message]
2022-04-29 15:35 ` Peter Gonda
2022-04-29 15:38 ` Paolo Bonzini
2022-04-29 15:51 ` Peter Gonda
2022-04-29 15:58 ` Paolo Bonzini
2022-04-29 17:12 ` Peter Gonda
2022-04-29 17:21 ` Paolo Bonzini
2022-04-29 17:27 ` Peter Gonda
2022-04-29 17:32 ` Paolo Bonzini
2022-04-29 17:33 ` Peter Gonda
[not found] ` <20220429010312.4013-1-hdanton@sina.com>
2022-04-29 8:48 ` Paolo Bonzini
[not found] ` <20220429114012.4127-1-hdanton@sina.com>
2022-04-29 13:44 ` Paolo Bonzini
[not found] ` <20220430015008.4257-1-hdanton@sina.com>
2022-04-30 8:11 ` Paolo Bonzini
2022-04-30 8:11 ` Paolo Bonzini
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