[PATCH v4 1/2] KVM: s390: pv: pull all vcpus out of SIE before converting to/from pv vcpu

Michael Mueller
2022-02-23T10:20:12Z

That will allow to modify the control blocks without locking the vcpus.
A kvm-unit-test surfaced a possible DEADLOCK situation when guest cpus are
executing intercepted instructions at this time.

[ 319.799638] ======================================================
[ 319.799639] WARNING: possible circular locking dependency detected
[ 319.799641] 5.17.0-rc5-08427-gfd14b6309198 #4661 Not tainted
[ 319.799643] ------------------------------------------------------
[ 319.799644] qemu-system-s39/14220 is trying to acquire lock:
[ 319.799646] 00000000b30c0b50 (&kvm->lock){+.+.}-{3:3}, at: kvm_s390_set_tod_clock+0x36/0x250
[ 319.799659]
but task is already holding lock:
[ 319.799660] 00000000b5beda60 (&vcpu->mutex){+.+.}-{3:3}, at: kvm_vcpu_ioctl+0x9a/0x958
[ 319.799665]
which lock already depends on the new lock.

[ 319.799667]
the existing dependency chain (in reverse order) is:
[ 319.799668]
-> #1 (&vcpu->mutex){+.+.}-{3:3}:
[ 319.799671] __mutex_lock+0x8a/0x798
[ 319.799677] mutex_lock_nested+0x32/0x40
[ 319.799679] kvm_arch_vm_ioctl+0x1902/0x2c58
[ 319.799682] kvm_vm_ioctl+0x5b0/0xa80
[ 319.799685] __s390x_sys_ioctl+0xbe/0x100
[ 319.799688] __do_syscall+0x1da/0x208
[ 319.799689] system_call+0x82/0xb0
[ 319.799692]
-> #0 (&kvm->lock){+.+.}-{3:3}:
[ 319.799694] __lock_acquire+0x1916/0x2e70
[ 319.799699] lock_acquire+0x164/0x388
[ 319.799702] __mutex_lock+0x8a/0x798
[ 319.799757] mutex_lock_nested+0x32/0x40
[ 319.799759] kvm_s390_set_tod_clock+0x36/0x250
[ 319.799761] kvm_s390_handle_b2+0x6cc/0x26f0
[ 319.799764] kvm_handle_sie_intercept+0x1fe/0xe98
[ 319.799765] kvm_arch_vcpu_ioctl_run+0xec8/0x1880
[ 319.799768] kvm_vcpu_ioctl+0x29e/0x958
[ 319.799769] __s390x_sys_ioctl+0xbe/0x100
[ 319.799771] __do_syscall+0x1da/0x208
[ 319.799773] system_call+0x82/0xb0
[ 319.799774]
other info that might help us debug this:

[ 319.799776] Possible unsafe locking scenario:

[ 319.799777] CPU0 CPU1
[ 319.799778] ---- ----
[ 319.799779] lock(&vcpu->mutex);
[ 319.799780] lock(&kvm->lock);
[ 319.799782] lock(&vcpu->mutex);
[ 319.799783] lock(&kvm->lock);
[ 319.799784]
*** DEADLOCK ***

[ 319.799785] 2 locks held by qemu-system-s39/14220:
[ 319.799787] #0: 00000000b5beda60 (&vcpu->mutex){+.+.}-{3:3}, at: kvm_vcpu_ioctl+0x9a/0x958
[ 319.799791] #1: 00000000b30c4588 (&kvm->srcu){....}-{0:0}, at: kvm_arch_vcpu_ioctl_run+0x6f2/0x1880
[ 319.799796]
stack backtrace:
[ 319.799798] CPU: 5 PID: 14220 Comm: qemu-system-s39 Not tainted 5.17.0-rc5-08427-gfd14b6309198 #4661
[ 319.799801] Hardware name: IBM 8561 T01 701 (LPAR)
[ 319.799802] Call Trace:
[ 319.799803] [<000000020d7410de>] dump_stack_lvl+0x76/0x98
[ 319.799808] [<000000020cbbd268>] check_noncircular+0x140/0x160
[ 319.799811] [<000000020cbc0efe>] __lock_acquire+0x1916/0x2e70
[ 319.799813] [<000000020cbc2dbc>] lock_acquire+0x164/0x388
[ 319.799816] [<000000020d75013a>] __mutex_lock+0x8a/0x798
[ 319.799818] [<000000020d75087a>] mutex_lock_nested+0x32/0x40
[ 319.799820] [<000000020cb029a6>] kvm_s390_set_tod_clock+0x36/0x250
[ 319.799823] [<000000020cb14d14>] kvm_s390_handle_b2+0x6cc/0x26f0
[ 319.799825] [<000000020cb09b6e>] kvm_handle_sie_intercept+0x1fe/0xe98
[ 319.799827] [<000000020cb06c28>] kvm_arch_vcpu_ioctl_run+0xec8/0x1880
[ 319.799829] [<000000020caeddc6>] kvm_vcpu_ioctl+0x29e/0x958
[ 319.799831] [<000000020ce4e82e>] __s390x_sys_ioctl+0xbe/0x100
[ 319.799833] [<000000020d744a72>] __do_syscall+0x1da/0x208
[ 319.799835] [<000000020d757322>] system_call+0x82/0xb0
[ 319.799836] INFO: lockdep is turned off.

Signed-off-by: Michael Mueller <mimu@linux.ibm.com>
---
arch/s390/kvm/kvm-s390.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 2296b1ff1e02..b75ea3e57172 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -2185,15 +2185,15 @@ static int kvm_s390_cpus_from_pv(struct kvm *kvm, u16 *rcp, u16 *rrcp)
* behind.
* We want to return the first failure rc and rrc, though.
*/
+ kvm_s390_vcpu_block_all(kvm);
kvm_for_each_vcpu(i, vcpu, kvm) {
- mutex_lock(&vcpu->mutex);
if (kvm_s390_pv_destroy_cpu(vcpu, &rc, &rrc) && !ret) {
*rcp = rc;
*rrcp = rrc;
ret = -EIO;
}
- mutex_unlock(&vcpu->mutex);
}
+ kvm_s390_vcpu_unblock_all(kvm);
return ret;
}

@@ -2205,13 +2205,13 @@ static int kvm_s390_cpus_to_pv(struct kvm *kvm, u16 *rc, u16 *rrc)

struct kvm_vcpu *vcpu;

+ kvm_s390_vcpu_block_all(kvm);
kvm_for_each_vcpu(i, vcpu, kvm) {
- mutex_lock(&vcpu->mutex);
r = kvm_s390_pv_create_cpu(vcpu, rc, rrc);
- mutex_unlock(&vcpu->mutex);
if (r)
break;
}
+ kvm_s390_vcpu_unblock_all(kvm);
if (r)
kvm_s390_cpus_from_pv(kvm, &dummy, &dummy);
return r;
--
2.32.0