Re: [PATCH] scsi: mpt3sas_scsih: remove unnecessary statics

[James Bottomley]

On Wed, 2017-07-19 at 17:06 -0500, Gustavo A. R. Silva wrote:
> Remove unnecessary static on local variables raid_device.
> Such variables are initialized before being used, on
> every execution path throughout the functions. The
> static has no benefit and, removing it reduces the
> object file size.
> 
> This issue was detected using Coccinelle and the following semantic
> patch:
> 
> @bad exists@
> position p;
> identifier x;
> type T;
> @@
> 
> static T x@p;
> ...
> x = <+...x...+>
> 
> @@
> identifier x;
> expression e;
> type T;
> position p != bad.p;
> @@
> 
> -static
> ÂT x@p;
> Â... when != x
> ÂÂÂÂÂwhen strict
> ?x = e;
> 
> In the following log you can see a significant difference in the
> object file size. This log is the output of the size command, before
> and after the code change:
> 
> before:
> ÂÂÂtextÂÂÂÂdataÂÂÂÂÂbssÂÂÂÂÂdecÂÂÂÂÂhex filename
> Â126304ÂÂÂ30384ÂÂÂÂ1280ÂÂ157968ÂÂÂ26910
> drivers/scsi/mpt3sas/mpt3sas_scsih.o
> 
> after:
> ÂÂÂtextÂÂÂÂdataÂÂÂÂÂbssÂÂÂÂÂdecÂÂÂÂÂhex filename
> Â126292ÂÂÂ30240ÂÂÂÂ1152ÂÂ157684ÂÂÂ267f4
> drivers/scsi/mpt3sas/mpt3sas_scsih.o

I've got to say I'm deeply uneasy about using a data/bss size reduction
as the benchmark for saying something shouldn't be declared static. ÂIn
this particular case the reduction is minimal, so it probably doesn't
matter; however, if the reduction were more significant, changing from
static to dynamic (i.e. on stack) allocation would increase the risk
that we'd blow the kernel stack. ÂIndeed one reason you might find
static declarations in functions is precisely to avoid blowing the
stack, so we wouldn't want to reverse them.

Other reasons for having static allocations instead of dynamic ones is
that you need to DMA to/from the structure (you can't DMA to stack) or
because you want to preserve values across function invocations.

There are definite reasons why statics in functions are a bad idea:
they prevent recursion and trip up code analysis, but in none of the
above cases would the fix be to remove the static qualifier.

James