Michael S. Tsirkin
2016-Aug-22 14:53 UTC
[PATCH] CodingStyle: add some more error handling guidelines
On Mon, Aug 22, 2016 at 08:16:17AM -0600, Jonathan Corbet wrote:> On Mon, 22 Aug 2016 16:57:46 +0300 > "Michael S. Tsirkin" <mst at redhat.com> wrote: > > > commit commit ea04036032edda6f771c1381d03832d2ed0f6c31 ("CodingStyle: > > add some more error handling guidelines") suggests never naming goto > > labels after the goto location - that is the error that is handled. > > > > But it's actually pretty common and IMHO it's a reasonable style > > provided each error gets its own label, and each label comes after the > > matching cleanup: > > > > foo = kmalloc(SIZE, GFP_KERNEL); > > if (!foo) > > goto err_foo; > > > > foo->bar = kmalloc(SIZE, GFP_KERNEL); > > if (!foo->bar) > > goto err_bar; > > ... > > > > kfree(foo->bar); > > err_bar: > > > > kfree(foo); > > err_foo: > > > > return ret; > > Hmm, I've never encountered that style, but I've never gone looking for it > either. I find it a little confusing to detach a label from the code it > will run. Is this really something we want to encourage? I kind of think > this one needs some acks before I can consider it.The point is really naming label for the part of init that failed (and so needs to be skipped), rather than the part that will run. Adding empty lines is not the point - does it look better like this? foo = kmalloc(SIZE, GFP_KERNEL); if (!foo) goto err_foo; foo->bar = kmalloc(SIZE, GFP_KERNEL); if (!foo->bar) goto err_bar; ... kfree(foo->bar); err_bar: kfree(foo); err_foo: return ret; I don't know whether there are examples outside code that I wrote myself, e.g. in vhost_dev_set_owner. I find that it's helpful since it avoids churn if more allocations are added.> > diff --git a/tools/virtio/ringtest/main.h b/tools/virtio/ringtest/main.h > > index 16917ac..e4d76c3 100644 > > --- a/tools/virtio/ringtest/main.h > > +++ b/tools/virtio/ringtest/main.h > > @@ -80,7 +80,9 @@ extern unsigned ring_size; > > > > /* Is there a portable way to do this? */ > > #if defined(__x86_64__) || defined(__i386__) > > -#define cpu_relax() asm ("rep; nop" ::: "memory") > > +#define cpu_relax() do { \ > > + asm ("rep; nop" ::: "memory"); \ > > +} while (0) > > #else > > #define cpu_relax() assert(0) > > #endif > > This hunk seems somehow unrelated, either that or I really haven't > understood the proposal :) > > jonOuch, that's unrelated, sorry. -- MST
Dan Carpenter
2016-Aug-22 18:31 UTC
[PATCH] CodingStyle: add some more error handling guidelines
vhost_dev_set_owner() is an example of why come-from labels are
bad style.
devel/drivers/vhost/vhost.c
473 /* Caller should have device mutex */
474 long vhost_dev_set_owner(struct vhost_dev *dev)
475 {
476 struct task_struct *worker;
477 int err;
478
479 /* Is there an owner already? */
480 if (vhost_dev_has_owner(dev)) {
481 err = -EBUSY;
482 goto err_mm;
What does goto err_mm do? It's actually a do-nothing goto. It would
be easier to read as a direct return. Why is it called err_mm? Because
originally the condition was:
if (dev->mm) {
err = -EBUSY;
goto err_mm;
}
We've changed the code but didn't update the label so it's slightly
confusing unless you know how vhost_dev_has_owner() is implemented.
483 }
484
485 /* No owner, become one */
486 dev->mm = get_task_mm(current);
487 worker = kthread_create(vhost_worker, dev, "vhost-%d",
current->pid);
488 if (IS_ERR(worker)) {
489 err = PTR_ERR(worker);
490 goto err_worker;
491 }
492
493 dev->worker = worker;
494 wake_up_process(worker); /* avoid contributing to loadavg
*/
495
496 err = vhost_attach_cgroups(dev);
497 if (err)
498 goto err_cgroup;
499
500 err = vhost_dev_alloc_iovecs(dev);
501 if (err)
502 goto err_cgroup;
This name doesn't make sense because it's a come-from label which is
used twice. Some people do:
if (err)
goto err_iovecs;
503
504 return 0;
Then they add two labels here:
err_iovecs:
err_cgroup:
kthread_stop(worker);
But if you base the label name on the label location then it makes
sense. goto stop_kthread; goto err_mmput;.
505 err_cgroup:
506 kthread_stop(worker);
507 dev->worker = NULL;
508 err_worker:
509 if (dev->mm)
510 mmput(dev->mm);
511 dev->mm = NULL;
512 err_mm:
513 return err;
514 }
regards,
dan carpenter
Michael S. Tsirkin
2016-Aug-22 18:39 UTC
[PATCH] CodingStyle: add some more error handling guidelines
On Mon, Aug 22, 2016 at 09:31:40PM +0300, Dan Carpenter wrote:> > vhost_dev_set_owner() is an example of why come-from labels are > bad style. > > devel/drivers/vhost/vhost.c > 473 /* Caller should have device mutex */ > 474 long vhost_dev_set_owner(struct vhost_dev *dev) > 475 { > 476 struct task_struct *worker; > 477 int err; > 478 > 479 /* Is there an owner already? */ > 480 if (vhost_dev_has_owner(dev)) { > 481 err = -EBUSY; > 482 goto err_mm; > > What does goto err_mm do? It's actually a do-nothing goto. It would > be easier to read as a direct return. Why is it called err_mm? Because > originally the condition was: > > if (dev->mm) { > err = -EBUSY; > goto err_mm; > } > > We've changed the code but didn't update the label so it's slightly > confusing unless you know how vhost_dev_has_owner() is implemented. > > 483 } > 484 > 485 /* No owner, become one */ > 486 dev->mm = get_task_mm(current); > 487 worker = kthread_create(vhost_worker, dev, "vhost-%d", current->pid); > 488 if (IS_ERR(worker)) { > 489 err = PTR_ERR(worker); > 490 goto err_worker; > 491 } > 492 > 493 dev->worker = worker; > 494 wake_up_process(worker); /* avoid contributing to loadavg */ > 495 > 496 err = vhost_attach_cgroups(dev); > 497 if (err) > 498 goto err_cgroup; > 499 > 500 err = vhost_dev_alloc_iovecs(dev); > 501 if (err) > 502 goto err_cgroup; > > This name doesn't make sense because it's a come-from label which is > used twice. Some people do: > > if (err) > goto err_iovecs; > > 503 > 504 return 0;Right and the current CodingStyle text seems to discourage this.> Then they add two labels here: > > err_iovecs: > err_cgroup: > kthread_stop(worker);Definitely good points above, I'll fix them up.> But if you base the label name on the label location then it makes > sense. goto stop_kthread; goto err_mmput;. > > 505 err_cgroup: > 506 kthread_stop(worker); > 507 dev->worker = NULL; > 508 err_worker: > 509 if (dev->mm) > 510 mmput(dev->mm); > 511 dev->mm = NULL; > 512 err_mm: > 513 return err; > 514 } > > regards, > dan carpenterOK, I'll consider this, thanks! -- MST
Dan Carpenter
2016-Aug-22 18:50 UTC
[PATCH] CodingStyle: add some more error handling guidelines
On Mon, Aug 22, 2016 at 05:53:02PM +0300, Michael S. Tsirkin wrote:> The point is really naming label for the part of init that failed > (and so needs to be skipped), rather than the part that will run.Naming labels after what "needs to be skipped" doesn't work. How does that meaning make sense for err_cgroup in vhost_dev_set_owner()? What needs to be skipped here? regards, dan carpenter
Michael S. Tsirkin
2016-Aug-22 19:31 UTC
[PATCH] CodingStyle: add some more error handling guidelines
On Mon, Aug 22, 2016 at 09:50:06PM +0300, Dan Carpenter wrote:> On Mon, Aug 22, 2016 at 05:53:02PM +0300, Michael S. Tsirkin wrote: > > The point is really naming label for the part of init that failed > > (and so needs to be skipped), rather than the part that will run. > > Naming labels after what "needs to be skipped" doesn't work. How does > that meaning make sense for err_cgroup in vhost_dev_set_owner()? What > needs to be skipped here? > > regards, > dan carpenterNothing because we are destroying the thread, so we don't need to detach it. I guess I'm convinced it's not very consistent at this point. -- MST
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