linux-kernel - Re: [PATCH 10/19] workqueue: update cwq alignement and make one more flag bit available

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Message-ID:  <87zl8bhs0s.fsf@blp.benpfaff.org>
Date:	Thu, 01 Oct 2009 11:45:55 -0700
From:	Ben Pfaff <blp@...stanford.edu>
To:	linux-kernel@...r.kernel.org
Subject:  Re: [PATCH 10/19] workqueue: update cwq alignement and make one more flag bit available

David Howells <dhowells@...hat.com> writes:

> Tejun Heo <tj@...nel.org> wrote:
>
>> +enum {
>> +	WORK_STRUCT_PENDING	= 0,	/* work item is pending execution */
>> +
>> +	/*
>> +	 * Reserve 3bits off of cwq pointer.  This is enough and
>> +	 * provides acceptable alignment on both 32 and 64bit
>> +	 * machines.
>> +	 */
>> +	WORK_STRUCT_FLAG_BITS	= 3,
>> +
>> +	WORK_STRUCT_FLAG_MASK	= (1UL << WORK_STRUCT_FLAG_BITS) - 1,
>> +	WORK_STRUCT_WQ_DATA_MASK = ~WORK_STRUCT_FLAG_MASK,
>> +};
>
> There's some great enum abuse going on here:-)

The "1UL" part is a bit worrisome.  Enumeration constants always
have type "int"[*], so if code that uses WORK_STRUCT_WQ_DATA_MASK
actually depends on getting a full "long" worth of bits, it is
not going to work on 64-bit systems.

[*] See C99:

     6.4.4.3 Enumeration constants
     Syntax
1              enumeration-constant:
                      identifier
     Semantics
2    An identifier declared as an enumeration constant has type int.

-- 
Ben Pfaff 
http://benpfaff.org

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