Re: [PATCH] Illustration of warning explosion silliness

Jeff Garzik
2006-09-28T01:48:47Z

Andrew Morton wrote:
> On Wed, 27 Sep 2006 20:58:30 -0400
> Jeff Garzik <jeff@garzik.org> wrote:
>
>> The following patch (DO NOT APPLY) illustrates why
>> device_for_each_child() should not be marked with __must_check.
>>
>> The function returns the return value of the actor function, and ceases
>> iteration upon error.
>>
>> However, _every_ case in drivers/scsi has a hardcoded return value,
>> illustrating how it is quite valid to not check the return value of this
>> function.
>>
>
> What does "has a hardcoded return value" mean?

Reference the sentence before that. The return value of the actor
passed to device_for_each_child() is always either zero (for some
actors) or one (for another actor). In all cases, it is never variable.

> AFICT the problem here is that (for example) (going up the call stack in
> the callee->caller direction):
>
> scsi_internal_device_block() returns an error code
>
> but device_block() drops that on the floor
>
> so target_block() drops it on the floor too
>
> so scsi_target_block() drops it on the floor too
>
>
> It's a small matter of (correct kernel) programming to correctly propagate
> the scsi_internal_device_block() error code all the way back out of
> scsi_target_block().
>
> It all looks rather sloppy?

Quite sloppy. But that doesn't change the fact that
device_for_each_child()'s actor _may_ hardcode the return value. It's a
valid usage model for that function.

If you are doing a simple collection of data -- adding items to a
preallocating list or bitmap -- or doing a search, as with
__remove_child() in drivers/scsi/scsi_sysfs.c, the return value can be
quite useless.

The usage model should not be _forced_ upon the caller, since it might
not be needed.

Jeff