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
