linux-kernel - Re: [PATCH] Illustration of warning explosion silliness

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Date:	Wed, 27 Sep 2006 21:48:42 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Andrew Morton <akpm@...l.org>
CC:	linux-scsi@...r.kernel.org, Greg KH <greg@...ah.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...l.org>
Subject: Re: [PATCH] Illustration of warning explosion silliness

Andrew Morton wrote:
> On Wed, 27 Sep 2006 20:58:30 -0400
> Jeff Garzik <jeff@...zik.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


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