Re: [PATCH 51/51] fs/zonefs: Fix sparse warnings in tracing code

[Bart Van Assche <bvanassche@acm.org>]

On 6/26/22 02:58, Luc Van Oostenryck wrote:
> On Sat, Jun 25, 2022 at 05:44:54PM -0700, Bart Van Assche wrote:
>> On 6/25/22 02:23, Christoph Hellwig wrote:
>>> Yeah, that is a bit of a mess.  Rasmus, Steven - any good idea how
>>> we can make the trace even macros fit for sparse?  Maybe just drop the
>>> is_signed_type check for __CHECKER__ ?
> 
> I would strongly advise against this:
> -) the macro is sued elsewhere too (for overflow checking)
> -) sparse wouldn't check anymore the same code as the one seen by the
>     compiler
> 
> What about I would add to sparse something to strip away the bitwise/
> recover the underlying type? Something like __unbitwiseof() or
> __underlying_typeof() (some better name is needed)?
> 
> Implementing directly what's needed here, something like __is_signed_type()
> would be possible too but is a bit too specialized and so much less useful.

Another question is how to keep the non-sparse build working. Does
anyone want to comment on the following alternatives or propose another
alternative?

(1) sparse implements __strip_bitwise as a macro.

(in compiler.h)

#ifndef __strip_bitwise
#define __strip_bitwise(type) type
#endif

(in trace_events.h)

#define is_signed_type(type) ((__strip_bitwise(type))(-1) < (__strip_bitwise(type))1)

(2) sparse implements __strip_bitwise as an operator that works on types.

#ifdef __CHECKER__
#define is_signed_type(type) ((__strip_bitwise(type))(-1) < (__strip_bitwise(type))1)
#else
#define is_signed_type(type) (((type)(-1)) < (type)1)
#endif

(1) would work better than (2) for kernel developers who are using a
version of sparse that does not support __strip_bitwise().

Thanks,

Bart.