[Qemu-devel] [PATCH] nvme: Make nvme_init error handling code more readable

[Qemu-devel] [PATCH] nvme: Make nvme_init error handling code more readable

[Fam Zheng]

Coverity doesn't like the tests under fail label (report CID 1385847).
Reset the fields so the clean up order is more apparent.

Signed-off-by: Fam Zheng <famz@redhat.com>
---
 block/nvme.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/block/nvme.c b/block/nvme.c
index 6f71122bf5..8239b920c8 100644
--- a/block/nvme.c
+++ b/block/nvme.c
@@ -560,6 +560,13 @@ static int nvme_init(BlockDriverState *bs, const char *device, int namespace,
     qemu_co_queue_init(&s->dma_flush_queue);
     s->nsid = namespace;
     s->aio_context = bdrv_get_aio_context(bs);
+
+    /* Fields we've not touched should be zero-initialized by block layer
+     * already, but reset them anyway to make the error handling code easier to
+     * reason. */
+    s->regs = NULL;
+    s->vfio = NULL;
+
     ret = event_notifier_init(&s->irq_notifier, 0);
     if (ret) {
         error_setg(errp, "Failed to init event notifier");
-- 
2.14.3

Re: [Qemu-devel] [PATCH] nvme: Make nvme_init error handling code more readable

[Peter Maydell]

On 21 May 2018 at 07:35, Fam Zheng <famz@redhat.com> wrote:
> Coverity doesn't like the tests under fail label (report CID 1385847).
> Reset the fields so the clean up order is more apparent.
>
> Signed-off-by: Fam Zheng <famz@redhat.com>
> ---
>  block/nvme.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/block/nvme.c b/block/nvme.c
> index 6f71122bf5..8239b920c8 100644
> --- a/block/nvme.c
> +++ b/block/nvme.c
> @@ -560,6 +560,13 @@ static int nvme_init(BlockDriverState *bs, const char *device, int namespace,
>      qemu_co_queue_init(&s->dma_flush_queue);
>      s->nsid = namespace;
>      s->aio_context = bdrv_get_aio_context(bs);
> +
> +    /* Fields we've not touched should be zero-initialized by block layer
> +     * already, but reset them anyway to make the error handling code easier to
> +     * reason. */
> +    s->regs = NULL;
> +    s->vfio = NULL;
> +
>      ret = event_notifier_init(&s->irq_notifier, 0);
>      if (ret) {
>          error_setg(errp, "Failed to init event notifier");

I suspect that either coverity or some compilers will complain that
the assignment to s->vfio here is redundant, because we'll either
return early without looking at it, or we'll get to the assignment
 s->vfio = qemu_vfio_open_pci(...)
which overrides it.

thanks
-- PMM

Re: [Qemu-devel] [PATCH] nvme: Make nvme_init error handling code more readable

[Fam Zheng]

On Mon, 05/21 09:35, Peter Maydell wrote:
> On 21 May 2018 at 07:35, Fam Zheng <famz@redhat.com> wrote:
> > Coverity doesn't like the tests under fail label (report CID 1385847).
> > Reset the fields so the clean up order is more apparent.
> >
> > Signed-off-by: Fam Zheng <famz@redhat.com>
> > ---
> >  block/nvme.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> >
> > diff --git a/block/nvme.c b/block/nvme.c
> > index 6f71122bf5..8239b920c8 100644
> > --- a/block/nvme.c
> > +++ b/block/nvme.c
> > @@ -560,6 +560,13 @@ static int nvme_init(BlockDriverState *bs, const char *device, int namespace,
> >      qemu_co_queue_init(&s->dma_flush_queue);
> >      s->nsid = namespace;
> >      s->aio_context = bdrv_get_aio_context(bs);
> > +
> > +    /* Fields we've not touched should be zero-initialized by block layer
> > +     * already, but reset them anyway to make the error handling code easier to
> > +     * reason. */
> > +    s->regs = NULL;
> > +    s->vfio = NULL;
> > +
> >      ret = event_notifier_init(&s->irq_notifier, 0);
> >      if (ret) {
> >          error_setg(errp, "Failed to init event notifier");
> 
> I suspect that either coverity or some compilers will complain that
> the assignment to s->vfio here is redundant, because we'll either
> return early without looking at it, or we'll get to the assignment
>  s->vfio = qemu_vfio_open_pci(...)
> which overrides it.

Hmm, okay, the compiler wants you to be at least as smart as it is, but no
smarter! I will revise the patch.

Fam

Re: [Qemu-devel] [PATCH] nvme: Make nvme_init error handling code more readable

[Paolo Bonzini]

On 21/05/2018 08:35, Fam Zheng wrote:
> Coverity doesn't like the tests under fail label (report CID 1385847).
> Reset the fields so the clean up order is more apparent.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> ---
>  block/nvme.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/block/nvme.c b/block/nvme.c
> index 6f71122bf5..8239b920c8 100644
> --- a/block/nvme.c
> +++ b/block/nvme.c
> @@ -560,6 +560,13 @@ static int nvme_init(BlockDriverState *bs, const char *device, int namespace,
>      qemu_co_queue_init(&s->dma_flush_queue);
>      s->nsid = namespace;
>      s->aio_context = bdrv_get_aio_context(bs);
> +
> +    /* Fields we've not touched should be zero-initialized by block layer
> +     * already, but reset them anyway to make the error handling code easier to
> +     * reason. */
> +    s->regs = NULL;
> +    s->vfio = NULL;
> +
>      ret = event_notifier_init(&s->irq_notifier, 0);
>      if (ret) {
>          error_setg(errp, "Failed to init event notifier");
> 

I think we should just mark it as a false positive or do something like

fail_regs:
    qemu_vfio_pci_unmap_bar(s->vfio, 0, (void *)s->regs, 0, NVME_BAR_SIZE);
fail_vfio:
    qemu_vfio_close(s->vfio);
fail:
    g_free(s->queues);
    event_notifier_cleanup(&s->irq_notifier);
    return ret;

even though it's a larger patch.

Paolo

Re: [Qemu-devel] [PATCH] nvme: Make nvme_init error handling code more readable

[Fam Zheng]

On Thu, 05/24 19:16, Paolo Bonzini wrote:
> On 21/05/2018 08:35, Fam Zheng wrote:
> > Coverity doesn't like the tests under fail label (report CID 1385847).
> > Reset the fields so the clean up order is more apparent.
> > 
> > Signed-off-by: Fam Zheng <famz@redhat.com>
> > ---
> >  block/nvme.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/block/nvme.c b/block/nvme.c
> > index 6f71122bf5..8239b920c8 100644
> > --- a/block/nvme.c
> > +++ b/block/nvme.c
> > @@ -560,6 +560,13 @@ static int nvme_init(BlockDriverState *bs, const char *device, int namespace,
> >      qemu_co_queue_init(&s->dma_flush_queue);
> >      s->nsid = namespace;
> >      s->aio_context = bdrv_get_aio_context(bs);
> > +
> > +    /* Fields we've not touched should be zero-initialized by block layer
> > +     * already, but reset them anyway to make the error handling code easier to
> > +     * reason. */
> > +    s->regs = NULL;
> > +    s->vfio = NULL;
> > +
> >      ret = event_notifier_init(&s->irq_notifier, 0);
> >      if (ret) {
> >          error_setg(errp, "Failed to init event notifier");
> > 
> 
> I think we should just mark it as a false positive or do something like
> 
> fail_regs:
>     qemu_vfio_pci_unmap_bar(s->vfio, 0, (void *)s->regs, 0, NVME_BAR_SIZE);
> fail_vfio:
>     qemu_vfio_close(s->vfio);
> fail:
>     g_free(s->queues);
>     event_notifier_cleanup(&s->irq_notifier);
>     return ret;
> 
> even though it's a larger patch.

And that makes five labels in total, I'm not sure I like it:

fail_handler:
    aio_set_event_notifier(bdrv_get_aio_context(bs), &s->irq_notifier,
                           false, NULL, NULL);
fail_queue:
    nvme_free_queue_pair(bs, s->queues[0]);
fail_regs:
    qemu_vfio_pci_unmap_bar(s->vfio, 0, (void *)s->regs, 0, NVME_BAR_SIZE);
fail_vfio:
    qemu_vfio_close(s->vfio);
fail:
    g_free(s->queues);
    event_notifier_cleanup(&s->irq_notifier);
    return ret;

Maybe we just mark it as false positive then?

Fam

Re: [Qemu-devel] [PATCH] nvme: Make nvme_init error handling code more readable

[Markus Armbruster]

Fam Zheng <famz@redhat.com> writes:

> On Thu, 05/24 19:16, Paolo Bonzini wrote:
>> On 21/05/2018 08:35, Fam Zheng wrote:
>> > Coverity doesn't like the tests under fail label (report CID 1385847).
>> > Reset the fields so the clean up order is more apparent.
>> > 
>> > Signed-off-by: Fam Zheng <famz@redhat.com>
>> > ---
>> >  block/nvme.c | 7 +++++++
>> >  1 file changed, 7 insertions(+)
>> > 
>> > diff --git a/block/nvme.c b/block/nvme.c
>> > index 6f71122bf5..8239b920c8 100644
>> > --- a/block/nvme.c
>> > +++ b/block/nvme.c
>> > @@ -560,6 +560,13 @@ static int nvme_init(BlockDriverState *bs, const char *device, int namespace,
>> >      qemu_co_queue_init(&s->dma_flush_queue);
>> >      s->nsid = namespace;
>> >      s->aio_context = bdrv_get_aio_context(bs);
>> > +
>> > +    /* Fields we've not touched should be zero-initialized by block layer
>> > +     * already, but reset them anyway to make the error handling code easier to
>> > +     * reason. */
>> > +    s->regs = NULL;
>> > +    s->vfio = NULL;
>> > +
>> >      ret = event_notifier_init(&s->irq_notifier, 0);
>> >      if (ret) {
>> >          error_setg(errp, "Failed to init event notifier");
>> > 
>> 
>> I think we should just mark it as a false positive or do something like
>> 
>> fail_regs:
>>     qemu_vfio_pci_unmap_bar(s->vfio, 0, (void *)s->regs, 0, NVME_BAR_SIZE);
>> fail_vfio:
>>     qemu_vfio_close(s->vfio);
>> fail:
>>     g_free(s->queues);
>>     event_notifier_cleanup(&s->irq_notifier);
>>     return ret;
>> 
>> even though it's a larger patch.
>
> And that makes five labels in total, I'm not sure I like it:
>
> fail_handler:
>     aio_set_event_notifier(bdrv_get_aio_context(bs), &s->irq_notifier,
>                            false, NULL, NULL);
> fail_queue:
>     nvme_free_queue_pair(bs, s->queues[0]);
> fail_regs:
>     qemu_vfio_pci_unmap_bar(s->vfio, 0, (void *)s->regs, 0, NVME_BAR_SIZE);
> fail_vfio:
>     qemu_vfio_close(s->vfio);
> fail:
>     g_free(s->queues);
>     event_notifier_cleanup(&s->irq_notifier);
>     return ret;

Doesn't look materially worse to me :)

With nice cleanup functions that detect "hasn't been set up" and do
nothing then, like free(NULL), you can use just one label.  Sadly,
cleanup functions are often not nice that way.

> Maybe we just mark it as false positive then?
>
> Fam

Re: [Qemu-devel] [PATCH] nvme: Make nvme_init error handling code more readable

[Fam Zheng]

On Fri, 05/25 07:47, Markus Armbruster wrote:
> Fam Zheng <famz@redhat.com> writes:
> 
> > On Thu, 05/24 19:16, Paolo Bonzini wrote:
> >> On 21/05/2018 08:35, Fam Zheng wrote:
> >> > Coverity doesn't like the tests under fail label (report CID 1385847).
> >> > Reset the fields so the clean up order is more apparent.
> >> > 
> >> > Signed-off-by: Fam Zheng <famz@redhat.com>
> >> > ---
> >> >  block/nvme.c | 7 +++++++
> >> >  1 file changed, 7 insertions(+)
> >> > 
> >> > diff --git a/block/nvme.c b/block/nvme.c
> >> > index 6f71122bf5..8239b920c8 100644
> >> > --- a/block/nvme.c
> >> > +++ b/block/nvme.c
> >> > @@ -560,6 +560,13 @@ static int nvme_init(BlockDriverState *bs, const char *device, int namespace,
> >> >      qemu_co_queue_init(&s->dma_flush_queue);
> >> >      s->nsid = namespace;
> >> >      s->aio_context = bdrv_get_aio_context(bs);
> >> > +
> >> > +    /* Fields we've not touched should be zero-initialized by block layer
> >> > +     * already, but reset them anyway to make the error handling code easier to
> >> > +     * reason. */
> >> > +    s->regs = NULL;
> >> > +    s->vfio = NULL;
> >> > +
> >> >      ret = event_notifier_init(&s->irq_notifier, 0);
> >> >      if (ret) {
> >> >          error_setg(errp, "Failed to init event notifier");
> >> > 
> >> 
> >> I think we should just mark it as a false positive or do something like
> >> 
> >> fail_regs:
> >>     qemu_vfio_pci_unmap_bar(s->vfio, 0, (void *)s->regs, 0, NVME_BAR_SIZE);
> >> fail_vfio:
> >>     qemu_vfio_close(s->vfio);
> >> fail:
> >>     g_free(s->queues);
> >>     event_notifier_cleanup(&s->irq_notifier);
> >>     return ret;
> >> 
> >> even though it's a larger patch.
> >
> > And that makes five labels in total, I'm not sure I like it:
> >
> > fail_handler:
> >     aio_set_event_notifier(bdrv_get_aio_context(bs), &s->irq_notifier,
> >                            false, NULL, NULL);
> > fail_queue:
> >     nvme_free_queue_pair(bs, s->queues[0]);
> > fail_regs:
> >     qemu_vfio_pci_unmap_bar(s->vfio, 0, (void *)s->regs, 0, NVME_BAR_SIZE);
> > fail_vfio:
> >     qemu_vfio_close(s->vfio);
> > fail:
> >     g_free(s->queues);
> >     event_notifier_cleanup(&s->irq_notifier);
> >     return ret;
> 
> Doesn't look materially worse to me :)

The labels themselves are not ugly or bad, but the goto statements above will be
harder to manage.

> 
> With nice cleanup functions that detect "hasn't been set up" and do
> nothing then, like free(NULL), you can use just one label.  Sadly,
> cleanup functions are often not nice that way.

nvme_free_queue_pair and qemu_vfio_close are cleanup functions and we can
improve them, but to make qemu_vfio_pci_unmap_bar behave similarly is just odd:
it's not a clean up function, at least not for s->vfio.

Fam

Re: [Qemu-devel] [PATCH] nvme: Make nvme_init error handling code more readable

[Markus Armbruster]

Fam Zheng <famz@redhat.com> writes:

> On Fri, 05/25 07:47, Markus Armbruster wrote:
>> Fam Zheng <famz@redhat.com> writes:
>> 
>> > On Thu, 05/24 19:16, Paolo Bonzini wrote:
>> >> On 21/05/2018 08:35, Fam Zheng wrote:
>> >> > Coverity doesn't like the tests under fail label (report CID 1385847).
>> >> > Reset the fields so the clean up order is more apparent.
>> >> > 
>> >> > Signed-off-by: Fam Zheng <famz@redhat.com>
>> >> > ---
>> >> >  block/nvme.c | 7 +++++++
>> >> >  1 file changed, 7 insertions(+)
>> >> > 
>> >> > diff --git a/block/nvme.c b/block/nvme.c
>> >> > index 6f71122bf5..8239b920c8 100644
>> >> > --- a/block/nvme.c
>> >> > +++ b/block/nvme.c
>> >> > @@ -560,6 +560,13 @@ static int nvme_init(BlockDriverState *bs, const char *device, int namespace,
>> >> >      qemu_co_queue_init(&s->dma_flush_queue);
>> >> >      s->nsid = namespace;
>> >> >      s->aio_context = bdrv_get_aio_context(bs);
>> >> > +
>> >> > +    /* Fields we've not touched should be zero-initialized by block layer
>> >> > +     * already, but reset them anyway to make the error handling code easier to
>> >> > +     * reason. */
>> >> > +    s->regs = NULL;
>> >> > +    s->vfio = NULL;
>> >> > +
>> >> >      ret = event_notifier_init(&s->irq_notifier, 0);
>> >> >      if (ret) {
>> >> >          error_setg(errp, "Failed to init event notifier");
>> >> > 
>> >> 
>> >> I think we should just mark it as a false positive or do something like
>> >> 
>> >> fail_regs:
>> >>     qemu_vfio_pci_unmap_bar(s->vfio, 0, (void *)s->regs, 0, NVME_BAR_SIZE);
>> >> fail_vfio:
>> >>     qemu_vfio_close(s->vfio);
>> >> fail:
>> >>     g_free(s->queues);
>> >>     event_notifier_cleanup(&s->irq_notifier);
>> >>     return ret;
>> >> 
>> >> even though it's a larger patch.
>> >
>> > And that makes five labels in total, I'm not sure I like it:
>> >
>> > fail_handler:
>> >     aio_set_event_notifier(bdrv_get_aio_context(bs), &s->irq_notifier,
>> >                            false, NULL, NULL);
>> > fail_queue:
>> >     nvme_free_queue_pair(bs, s->queues[0]);
>> > fail_regs:
>> >     qemu_vfio_pci_unmap_bar(s->vfio, 0, (void *)s->regs, 0, NVME_BAR_SIZE);
>> > fail_vfio:
>> >     qemu_vfio_close(s->vfio);
>> > fail:
>> >     g_free(s->queues);
>> >     event_notifier_cleanup(&s->irq_notifier);
>> >     return ret;
>> 
>> Doesn't look materially worse to me :)
>
> The labels themselves are not ugly or bad, but the goto statements above will be
> harder to manage.

Slightly.  The difference between three and five feels smaller than say
the one between one and three.  Admittedly subjective.

>> With nice cleanup functions that detect "hasn't been set up" and do
>> nothing then, like free(NULL), you can use just one label.  Sadly,
>> cleanup functions are often not nice that way.
>
> nvme_free_queue_pair and qemu_vfio_close are cleanup functions and we can
> improve them, but to make qemu_vfio_pci_unmap_bar behave similarly is just odd:
> it's not a clean up function, at least not for s->vfio.

The technique isn't "all or nothing".  Reducing the number of labels is
nice even when you can't reduce them to one.

Re: [Qemu-devel] [PATCH] nvme: Make nvme_init error handling code more readable

[Eric Blake]

On 05/25/2018 01:25 AM, Fam Zheng wrote:

>>> And that makes five labels in total, I'm not sure I like it:
>>>
>>> fail_handler:
>>>      aio_set_event_notifier(bdrv_get_aio_context(bs), &s->irq_notifier,
>>>                             false, NULL, NULL);
>>> fail_queue:
>>>      nvme_free_queue_pair(bs, s->queues[0]);
>>> fail_regs:
>>>      qemu_vfio_pci_unmap_bar(s->vfio, 0, (void *)s->regs, 0, NVME_BAR_SIZE);
>>> fail_vfio:
>>>      qemu_vfio_close(s->vfio);
>>> fail:
>>>      g_free(s->queues);
>>>      event_notifier_cleanup(&s->irq_notifier);
>>>      return ret;
>>
>> Doesn't look materially worse to me :)
> 
> The labels themselves are not ugly or bad, but the goto statements above will be
> harder to manage.
> 
>>
>> With nice cleanup functions that detect "hasn't been set up" and do
>> nothing then, like free(NULL), you can use just one label.  Sadly,
>> cleanup functions are often not nice that way.
> 
> nvme_free_queue_pair and qemu_vfio_close are cleanup functions and we can
> improve them, but to make qemu_vfio_pci_unmap_bar behave similarly is just odd:
> it's not a clean up function, at least not for s->vfio.

But even then, you can do:

fail:
     if (s->vfio) {
         qemu_vfio_close(s->vfio);
     }

That is, there are ways to make a single cleanup path more applicable, 
regardless of where you decided you needed an early cleanup.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org