Currently the calculations of stolen time for PPC Book3S HV guests
uses fields in both the vcpu struct and the kvmppc_vcore struct. The
fields in the kvmppc_vcore struct are protected by the
vcpu->arch.tbacct_lock of the vcpu that has taken responsibility for
running the virtual core. This works correctly but confuses lockdep,
because it sees that the code takes the tbacct_lock for a vcpu in
kvmppc_remove_runnable() and then takes another vcpu's tbacct_lock in
vcore_stolen_time(), and it thinks there is a possibility of deadlock,
causing it to print reports like this:
=============================================
[ INFO: possible recursive locking detected ]
3.18.0-rc7-kvm-00016-g8db4bc6 #89 Not tainted
---------------------------------------------
qemu-system-ppc/6188 is trying to acquire lock:
(&(&vcpu->arch.tbacct_lock)->rlock){......}, at: [<d00000000ecb1fe8>] .vcore_stolen_time+0x48/0xd0 [kvm_hv]
but task is already holding lock:
(&(&vcpu->arch.tbacct_lock)->rlock){......}, at: [<d00000000ecb25a0>] .kvmppc_remove_runnable.part.3+0x30/0xd0 [kvm_hv]
other info that might help us debug this:
Possible unsafe locking scenario:
CPU0
----
lock(&(&vcpu->arch.tbacct_lock)->rlock);
lock(&(&vcpu->arch.tbacct_lock)->rlock);
*** DEADLOCK ***
May be due to missing lock nesting notation
3 locks held by qemu-system-ppc/6188:
#0: (&vcpu->mutex){+.+.+.}, at: [<d00000000eb93f98>] .vcpu_load+0x28/0xe0 [kvm]
#1: (&(&vcore->lock)->rlock){+.+...}, at: [<d00000000ecb41b0>] .kvmppc_vcpu_run_hv+0x530/0x1530 [kvm_hv]
#2: (&(&vcpu->arch.tbacct_lock)->rlock){......}, at: [<d00000000ecb25a0>] .kvmppc_remove_runnable.part.3+0x30/0xd0 [kvm_hv]
stack backtrace:
CPU: 40 PID: 6188 Comm: qemu-system-ppc Not tainted 3.18.0-rc7-kvm-00016-g8db4bc6 #89
Call Trace:
[c000000b2754f3f0] [c000000000b31b6c] .dump_stack+0x88/0xb4 (unreliable)
[c000000b2754f470] [c0000000000faeb8] .__lock_acquire+0x1878/0x2190
[c000000b2754f600] [c0000000000fbf0c] .lock_acquire+0xcc/0x1a0
[c000000b2754f6d0] [c000000000b2954c] ._raw_spin_lock_irq+0x4c/0x70
[c000000b2754f760] [d00000000ecb1fe8] .vcore_stolen_time+0x48/0xd0 [kvm_hv]
[c000000b2754f7f0] [d00000000ecb25b4] .kvmppc_remove_runnable.part.3+0x44/0xd0 [kvm_hv]
[c000000b2754f880] [d00000000ecb43ec] .kvmppc_vcpu_run_hv+0x76c/0x1530 [kvm_hv]
[c000000b2754f9f0] [d00000000eb9f46c] .kvmppc_vcpu_run+0x2c/0x40 [kvm]
[c000000b2754fa60] [d00000000eb9c9a4] .kvm_arch_vcpu_ioctl_run+0x54/0x160 [kvm]
[c000000b2754faf0] [d00000000eb94538] .kvm_vcpu_ioctl+0x498/0x760 [kvm]
[c000000b2754fcb0] [c000000000267eb4] .do_vfs_ioctl+0x444/0x770
[c000000b2754fd90] [c0000000002682a4] .SyS_ioctl+0xc4/0xe0
[c000000b2754fe30] [c0000000000092e4] syscall_exit+0x0/0x98
In order to make the locking easier to analyse, we change the code to
use a spinlock in the kvmppc_vcore struct to protect the stolen_tb and
preempt_tb fields. This lock needs to be an irq-safe lock since it is
used in the kvmppc_core_vcpu_load_hv() and kvmppc_core_vcpu_put_hv()
functions, which are called with the scheduler rq lock held, which is
an irq-safe lock.
Signed-off-by: Paul Mackerras <paulus@xxxxxxxxx>
---
arch/powerpc/include/asm/kvm_host.h | 1 +
arch/powerpc/kvm/book3s_hv.c | 60 +++++++++++++++++++------------------
2 files changed, 32 insertions(+), 29 deletions(-)
diff --git a/arch/powerpc/include/asm/kvm_host.h b/arch/powerpc/include/asm/kvm_host.h
index 19ff9ee..63a66dd 100644
--- a/arch/powerpc/include/asm/kvm_host.h
+++ b/arch/powerpc/include/asm/kvm_host.h
@@ -278,6 +278,7 @@ struct kvmppc_vcore {
struct list_head runnable_threads;
spinlock_t lock;
wait_queue_head_t wq;
+ spinlock_t stoltb_lock; /* protects stolen_tb and preempt_tb */
u64 stolen_tb;
u64 preempt_tb;
struct kvm_vcpu *runner;
diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index b404cc6..02fbf5d 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -135,11 +135,10 @@ static void kvmppc_fast_vcpu_kick_hv(struct kvm_vcpu *vcpu)
* stolen.
*
* Updates to busy_stolen are protected by arch.tbacct_lock;
- * updates to vc->stolen_tb are protected by the arch.tbacct_lock
- * of the vcpu that has taken responsibility for running the vcore
- * (i.e. vc->runner). The stolen times are measured in units of
- * timebase ticks. (Note that the != TB_NIL checks below are
- * purely defensive; they should never fail.)
+ * updates to vc->stolen_tb are protected by the vcore->stoltb_lock
+ * lock. The stolen times are measured in units of timebase ticks.
+ * (Note that the != TB_NIL checks below are purely defensive;
+ * they should never fail.)
*/
static void kvmppc_core_vcpu_load_hv(struct kvm_vcpu *vcpu, int cpu)
@@ -147,12 +146,21 @@ static void kvmppc_core_vcpu_load_hv(struct kvm_vcpu *vcpu, int cpu)
struct kvmppc_vcore *vc = vcpu->arch.vcore;
unsigned long flags;
- spin_lock_irqsave(&vcpu->arch.tbacct_lock, flags);
- if (vc->runner == vcpu && vc->vcore_state != VCORE_INACTIVE &&
- vc->preempt_tb != TB_NIL) {
- vc->stolen_tb += mftb() - vc->preempt_tb;
- vc->preempt_tb = TB_NIL;
+ /*
+ * We can test vc->runner without taking the vcore lock,
+ * because only this task ever sets vc->runner to this
+ * vcpu, and once it is set to this vcpu, only this task
+ * ever sets it to NULL.
+ */
+ if (vc->runner == vcpu && vc->vcore_state != VCORE_INACTIVE) {
+ spin_lock_irqsave(&vc->stoltb_lock, flags);
+ if (vc->preempt_tb != TB_NIL) {
+ vc->stolen_tb += mftb() - vc->preempt_tb;
+ vc->preempt_tb = TB_NIL;
+ }
+ spin_unlock_irqrestore(&vc->stoltb_lock, flags);
}
+ spin_lock_irqsave(&vcpu->arch.tbacct_lock, flags);
if (vcpu->arch.state == KVMPPC_VCPU_BUSY_IN_HOST &&
vcpu->arch.busy_preempt != TB_NIL) {
vcpu->arch.busy_stolen += mftb() - vcpu->arch.busy_preempt;
@@ -166,9 +174,12 @@ static void kvmppc_core_vcpu_put_hv(struct kvm_vcpu *vcpu)
struct kvmppc_vcore *vc = vcpu->arch.vcore;
unsigned long flags;
- spin_lock_irqsave(&vcpu->arch.tbacct_lock, flags);
- if (vc->runner == vcpu && vc->vcore_state != VCORE_INACTIVE)
+ if (vc->runner == vcpu && vc->vcore_state != VCORE_INACTIVE) {
+ spin_lock_irqsave(&vc->stoltb_lock, flags);
vc->preempt_tb = mftb();
+ spin_unlock_irqrestore(&vc->stoltb_lock, flags);
+ }
+ spin_lock_irqsave(&vcpu->arch.tbacct_lock, flags);
if (vcpu->arch.state == KVMPPC_VCPU_BUSY_IN_HOST)
vcpu->arch.busy_preempt = mftb();
spin_unlock_irqrestore(&vcpu->arch.tbacct_lock, flags);
@@ -502,25 +513,14 @@ static void kvmppc_update_vpas(struct kvm_vcpu *vcpu)
static u64 vcore_stolen_time(struct kvmppc_vcore *vc, u64 now)
{
u64 p;
+ unsigned long flags;
- /*
- * If we are the task running the vcore, then since we hold
- * the vcore lock, we can't be preempted, so stolen_tb/preempt_tb
- * can't be updated, so we don't need the tbacct_lock.
- * If the vcore is inactive, it can't become active (since we
- * hold the vcore lock), so the vcpu load/put functions won't
- * update stolen_tb/preempt_tb, and we don't need tbacct_lock.
- */
+ spin_lock_irqsave(&vc->stoltb_lock, flags);
+ p = vc->stolen_tb;
if (vc->vcore_state != VCORE_INACTIVE &&
- vc->runner->arch.run_task != current) {
- spin_lock_irq(&vc->runner->arch.tbacct_lock);
- p = vc->stolen_tb;
- if (vc->preempt_tb != TB_NIL)
- p += now - vc->preempt_tb;
- spin_unlock_irq(&vc->runner->arch.tbacct_lock);
- } else {
- p = vc->stolen_tb;
- }
+ vc->preempt_tb != TB_NIL)
+ p += now - vc->preempt_tb;
+ spin_unlock_irqrestore(&vc->stoltb_lock, flags);
return p;
}
@@ -1390,6 +1390,7 @@ static struct kvmppc_vcore *kvmppc_vcore_create(struct kvm *kvm, int core)
INIT_LIST_HEAD(&vcore->runnable_threads);
spin_lock_init(&vcore->lock);
+ spin_lock_init(&vcore->stoltb_lock);
init_waitqueue_head(&vcore->wq);
vcore->preempt_tb = TB_NIL;
vcore->lpcr = kvm->arch.lpcr;
@@ -1727,6 +1728,7 @@ static void kvmppc_run_core(struct kvmppc_vcore *vc)
vc->n_woken = 0;
vc->nap_count = 0;
vc->entry_exit_count = 0;
+ vc->preempt_tb = TB_NIL;
vc->vcore_state = VCORE_STARTING;
vc->in_guest = 0;
vc->napping_threads = 0;
--
2.1.3
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