linux-kernel - Re: [PATCH] Documentation: kvm: fix SRCU locking order docs

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Date:   Fri, 13 Jan 2023 10:20:23 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Boqun Feng <boqun.feng@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        seanjc@...gle.com, Joel Fernandes <joel@...lfernandes.org>,
        Matthew Wilcox <willy@...radead.org>,
        Josh Triplett <josh@...htriplett.org>, rcu@...r.kernel.org,
        Michal Luczaj <mhal@...x.co>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] Documentation: kvm: fix SRCU locking order docs

On 1/13/23 08:18, Boqun Feng wrote:
> On Thu, Jan 12, 2023 at 07:20:48AM -0800, Paul E. McKenney wrote:
>> On Thu, Jan 12, 2023 at 08:24:16AM +0000, David Woodhouse wrote:
>>> On Wed, 2023-01-11 at 13:30 -0500, Paolo Bonzini wrote:
>>>>
>>>> +- ``synchronize_srcu(&kvm->srcu)`` is called inside critical sections
>>>> +  for kvm->lock, vcpu->mutex and kvm->slots_lock.  These locks _cannot_
>>>> +  be taken inside a kvm->srcu read-side critical section; that is, the
>>>> +  following is broken::
>>>> +
>>>> +      srcu_read_lock(&kvm->srcu);
>>>> +      mutex_lock(&kvm->slots_lock);
>>>> +
>>>
>>> "Don't tell me. Tell lockdep!"
>>>
>>> Did we conclude in
>>> https://lore.kernel.org/kvm/122f38e724aae9ae8ab474233da1ba19760c20d2.camel@infradead.org/
>>> that lockdep *could* be clever enough to catch a violation of this rule
>>> by itself?
>>>
>>> The general case of the rule would be that 'if mutex A is taken in a
>>> read-section for SCRU B, then any synchronize_srcu(B) while mutex A is
>>> held shall be verboten'. And vice versa.
>>>
>>> If we can make lockdep catch it automatically, yay!
>>
>> Unfortunately, lockdep needs to see a writer to complain, and that patch
>> just adds a reader.  And adding that writer would make lockdep complain
>> about things that are perfectly fine.  It should be possible to make
>> lockdep catch this sort of thing, but from what I can see, doing so
>> requires modifications to lockdep itself.
>>
> 
> Please see if the follow patchset works:
> 
> 	https://lore.kernel.org/lkml/20230113065955.815667-1-boqun.feng@gmail.com
> 
> "I have been called. I must answer. Always." ;-)

It's missing an important testcase; if it passes (does not warn), then 
it should work:

CPU 1                                    CPU 2
----------------------------             ------------------------------
mutex_lock(&m1);                         srcu_read_lock(&srcu1);
srcu_read_lock(&srcu1);                  mutex_lock(&m1);
srcu_read_unlock(&srcu1);                mutex_unlock(&m1);
mutex_unlock(&m1);                       srcu_read_unlock(&srcu1);

This is the main difference, lockdep-wise, between SRCU and an rwlock.

Paolo