[PATCH] virt: kvm: arm: vgic: Return failure code '-EBUSY' when mutex_trylock() fails

[PATCH] virt: kvm: arm: vgic: Return failure code '-EBUSY' when mutex_trylock() fails

[Chen Gang]

When mutex_trylock() fails, kvm_vgic_create() will not create 'vgic', so
it need return failure code '-EBUSY' instead of '0' to let outside know
about it.

Also simplify the code within kvm_vgic_create():

 - kvm_for_each_vcpu() scanning once is enough for current case.

 - Remove redundant variable 'vcpu_lock_idx'.


Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
---
 virt/kvm/arm/vgic.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/virt/kvm/arm/vgic.c b/virt/kvm/arm/vgic.c
index 3aaca49..5846725 100644
--- a/virt/kvm/arm/vgic.c
+++ b/virt/kvm/arm/vgic.c
@@ -1933,7 +1933,7 @@ out:
 
 int kvm_vgic_create(struct kvm *kvm)
 {
-	int i, vcpu_lock_idx = -1, ret = 0;
+	int i, ret = 0;
 	struct kvm_vcpu *vcpu;
 
 	mutex_lock(&kvm->lock);
@@ -1949,13 +1949,12 @@ int kvm_vgic_create(struct kvm *kvm)
 	 * that no other VCPUs are run while we create the vgic.
 	 */
 	kvm_for_each_vcpu(i, vcpu, kvm) {
-		if (!mutex_trylock(&vcpu->mutex))
+		if (!mutex_trylock(&vcpu->mutex)) {
+			ret = -EBUSY;
 			goto out_unlock;
-		vcpu_lock_idx = i;
-	}
-
-	kvm_for_each_vcpu(i, vcpu, kvm) {
+		}
 		if (vcpu->arch.has_run_once) {
+			mutex_unlock(&vcpu->mutex);
 			ret = -EBUSY;
 			goto out_unlock;
 		}
@@ -1968,8 +1967,8 @@ int kvm_vgic_create(struct kvm *kvm)
 	kvm->arch.vgic.vgic_cpu_base = VGIC_ADDR_UNDEF;
 
 out_unlock:
-	for (; vcpu_lock_idx >= 0; vcpu_lock_idx--) {
-		vcpu = kvm_get_vcpu(kvm, vcpu_lock_idx);
+	while (i-- > 0) {
+		vcpu = kvm_get_vcpu(kvm, i);
 		mutex_unlock(&vcpu->mutex);
 	}
 
-- 
1.9.3

Re: [PATCH] virt: kvm: arm: vgic: Return failure code '-EBUSY' when mutex_trylock() fails

[Christoffer Dall]

On Wed, Nov 12, 2014 at 11:04:23PM +0800, Chen Gang wrote:
> When mutex_trylock() fails, kvm_vgic_create() will not create 'vgic', so
> it need return failure code '-EBUSY' instead of '0' to let outside know
> about it.

I already sent a patch for the -EBUSY:
https://lists.cs.columbia.edu/pipermail/kvmarm/2014-November/011936.html

> 
> Also simplify the code within kvm_vgic_create():
> 
>  - kvm_for_each_vcpu() scanning once is enough for current case.
> 
>  - Remove redundant variable 'vcpu_lock_idx'.

I don't like using the iterator variable for this kind of thing because
it can really break in languages where i is out-of-scope after the loop
and it is too easy to reuse the iterator variable in later versions of
the code.

That being said, the scanning once is slightly prettier I guess,
but I'd rather not introduce the churn unless others (Marc) think this
is a big win.

-Christoffer

> 
> 
> Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
> ---
>  virt/kvm/arm/vgic.c | 15 +++++++--------
>  1 file changed, 7 insertions(+), 8 deletions(-)
> 
> diff --git a/virt/kvm/arm/vgic.c b/virt/kvm/arm/vgic.c
> index 3aaca49..5846725 100644
> --- a/virt/kvm/arm/vgic.c
> +++ b/virt/kvm/arm/vgic.c
> @@ -1933,7 +1933,7 @@ out:
>  
>  int kvm_vgic_create(struct kvm *kvm)
>  {
> -	int i, vcpu_lock_idx = -1, ret = 0;
> +	int i, ret = 0;
>  	struct kvm_vcpu *vcpu;
>  
>  	mutex_lock(&kvm->lock);
> @@ -1949,13 +1949,12 @@ int kvm_vgic_create(struct kvm *kvm)
>  	 * that no other VCPUs are run while we create the vgic.
>  	 */
>  	kvm_for_each_vcpu(i, vcpu, kvm) {
> -		if (!mutex_trylock(&vcpu->mutex))
> +		if (!mutex_trylock(&vcpu->mutex)) {
> +			ret = -EBUSY;
>  			goto out_unlock;
> -		vcpu_lock_idx = i;
> -	}
> -
> -	kvm_for_each_vcpu(i, vcpu, kvm) {
> +		}
>  		if (vcpu->arch.has_run_once) {
> +			mutex_unlock(&vcpu->mutex);
>  			ret = -EBUSY;
>  			goto out_unlock;
>  		}
> @@ -1968,8 +1967,8 @@ int kvm_vgic_create(struct kvm *kvm)
>  	kvm->arch.vgic.vgic_cpu_base = VGIC_ADDR_UNDEF;
>  
>  out_unlock:
> -	for (; vcpu_lock_idx >= 0; vcpu_lock_idx--) {
> -		vcpu = kvm_get_vcpu(kvm, vcpu_lock_idx);
> +	while (i-- > 0) {
> +		vcpu = kvm_get_vcpu(kvm, i);
>  		mutex_unlock(&vcpu->mutex);
>  	}
>  
> -- 
> 1.9.3

Re: [PATCH] virt: kvm: arm: vgic: Return failure code '-EBUSY' when mutex_trylock() fails

[Chen Gang]

On 11/13/14 3:41, Christoffer Dall wrote:
> On Wed, Nov 12, 2014 at 11:04:23PM +0800, Chen Gang wrote:
>> When mutex_trylock() fails, kvm_vgic_create() will not create 'vgic', so
>> it need return failure code '-EBUSY' instead of '0' to let outside know
>> about it.
> 
> I already sent a patch for the -EBUSY:
> https://lists.cs.columbia.edu/pipermail/kvmarm/2014-November/011936.html
> 

Yeah, really it is.

>>
>> Also simplify the code within kvm_vgic_create():
>>
>>  - kvm_for_each_vcpu() scanning once is enough for current case.
>>
>>  - Remove redundant variable 'vcpu_lock_idx'.
> 
> I don't like using the iterator variable for this kind of thing because
> it can really break in languages where i is out-of-scope after the loop
> and it is too easy to reuse the iterator variable in later versions of
> the code.
> 

For me, what you said is OK, we can still keep it no touch.

> That being said, the scanning once is slightly prettier I guess,
> but I'd rather not introduce the churn unless others (Marc) think this
> is a big win.
> 

If only merge the 2 scanning loops, it will not change much. And also
can let code simpler and clearer for readers (both are for processing
and checking '-EBUSY').

If possible, after your patch merges into linux next tree, I will send
the related improving patch for it.

Thanks.

> -Christoffer
> 
>>
>>
>> Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
>> ---
>>  virt/kvm/arm/vgic.c | 15 +++++++--------
>>  1 file changed, 7 insertions(+), 8 deletions(-)
>>
>> diff --git a/virt/kvm/arm/vgic.c b/virt/kvm/arm/vgic.c
>> index 3aaca49..5846725 100644
>> --- a/virt/kvm/arm/vgic.c
>> +++ b/virt/kvm/arm/vgic.c
>> @@ -1933,7 +1933,7 @@ out:
>>  
>>  int kvm_vgic_create(struct kvm *kvm)
>>  {
>> -	int i, vcpu_lock_idx = -1, ret = 0;
>> +	int i, ret = 0;
>>  	struct kvm_vcpu *vcpu;
>>  
>>  	mutex_lock(&kvm->lock);
>> @@ -1949,13 +1949,12 @@ int kvm_vgic_create(struct kvm *kvm)
>>  	 * that no other VCPUs are run while we create the vgic.
>>  	 */
>>  	kvm_for_each_vcpu(i, vcpu, kvm) {
>> -		if (!mutex_trylock(&vcpu->mutex))
>> +		if (!mutex_trylock(&vcpu->mutex)) {
>> +			ret = -EBUSY;
>>  			goto out_unlock;
>> -		vcpu_lock_idx = i;
>> -	}
>> -
>> -	kvm_for_each_vcpu(i, vcpu, kvm) {
>> +		}
>>  		if (vcpu->arch.has_run_once) {
>> +			mutex_unlock(&vcpu->mutex);
>>  			ret = -EBUSY;
>>  			goto out_unlock;
>>  		}
>> @@ -1968,8 +1967,8 @@ int kvm_vgic_create(struct kvm *kvm)
>>  	kvm->arch.vgic.vgic_cpu_base = VGIC_ADDR_UNDEF;
>>  
>>  out_unlock:
>> -	for (; vcpu_lock_idx >= 0; vcpu_lock_idx--) {
>> -		vcpu = kvm_get_vcpu(kvm, vcpu_lock_idx);
>> +	while (i-- > 0) {
>> +		vcpu = kvm_get_vcpu(kvm, i);
>>  		mutex_unlock(&vcpu->mutex);
>>  	}
>>  
>> -- 
>> 1.9.3

-- 
Chen Gang

Open, share, and attitude like air, water, and life which God blessed