Hi!
KVM on ARM has this function, and it seems to be only used in a couple of places, mostly for
initialization.
We recently noticed a CI failure roughly like that:
[ 328.171264] BUG: MAX_LOCK_DEPTH too low!
[ 328.175227] turning off the locking correctness validator.
[ 328.180726] Please attach the output of /proc/lock_stat to the bug report
[ 328.187531] depth: 48 max: 48!
[ 328.190678] 48 locks held by qemu-kvm/11664:
[ 328.194957] #0: ffff800086de5ba0 (&kvm->lock){+.+.}-{3:3}, at: kvm_ioctl_create_device+0x174/0x5b0
[ 328.204048] #1: ffff0800e78800b8 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
[ 328.212521] #2: ffff07ffeee51e98 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
[ 328.220991] #3: ffff0800dc7d80b8 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
[ 328.229463] #4: ffff07ffe0c980b8 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
[ 328.237934] #5: ffff0800a3883c78 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
[ 328.246405] #6: ffff07fffbe480b8 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
..
..
..
..
As far as I see currently MAX_LOCK_DEPTH is 48 and the number of vCPUs can easily be hundreds.
Do you think that it's possible? or know if there were any efforts to get rid of lock_all_vcpus to avoid
this problem? If not possible, maybe we can exclude the lock_all_vcpus from the lockdep validator?
AFAIK, on x86 most of the similar cases where lock_all_vcpus could be used are handled by
assuming and enforcing that userspace will call these functions prior to first vCPU is created an/or run,
thus the need for such locking doesn't exist.
Recently x86 got a lot of cleanups to enforce this, like for example enforce that userspace won't change
CPUID after a vCPU has run.
Best regards,
Maxim Levitsky
