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

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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@xxxxxxxxxxxxx/
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@xxxxxxxxx

"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




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