Messages in this thread | | | Subject | Re: [PATCH] cred: Remove unnecessary kdebug atomic reads | From | Joe Perches <> | Date | Tue, 25 Aug 2015 14:10:03 -0700 |
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On Tue, 2015-08-25 at 13:56 -0700, Andrew Morton wrote: > On Tue, 25 Aug 2015 13:51:06 -0700 Joe Perches <joe@perches.com> wrote: > > > On Tue, 2015-08-25 at 13:39 -0700, Andrew Morton wrote: > > > On Tue, 25 Aug 2015 09:53:51 -0700 Joe Perches <joe@perches.com> wrote: > > > > commit e0e817392b9a ("CRED: Add some configurable debugging [try #6]") > > > > added the kdebug mechanism to this file back in 2009. > > > > > > > > The kdebug macro calls no_printk which always evaluates arguments. > > > > > > > > Most of the kdebug uses have an unnecessary call of > > > > atomic_read(&cred->usage) > > > > > > > > Make the kdebug macro do nothing by defining it with > > > > do { if (0) no_printk(...); } while (0) > > > > when not enabled. > > [] > > > Did you consider doing this within no_printk()? > > > > Yes. > > > > > That would break code > > > which is relying on side-effects in the evaluation of a printk arg but > > > that's pretty weird and I bet there isn't (and won't be) such code. > > > > I'll bet you there is more than a little and I don't want to > > experiment with it unconditionally. > > > > All printks would need to be evaluated for that side-effect. > > > > Safer would be to create a new no_eval_printk macro and convert > > the no_printk uses over to that as appropriate and possibly create > > a CONFIG_ option to use no_eval_printk instead of no_printk/printk > > and let the adventurous find the side-effects. > > > > Maybe a coccinelle script can be written to find all the locations > > with evaluated non-constant expression arguments with side-effects. > > wimp.
twice shy...
Coccinelle isn't very good at calling tree analysis, so it'd be a difficult thing for it to do well anyway.
btw; I seems to recall suggestions around the same thing when no_printk was moved from subsystems to kernel.h by David Howells in 2010.
> That duplicated printk in cred.c is nasty. We could do this? > > #if 0 > #define __kdebug printk > #else > #define __kdebug if (0) no_printk > #endif
You could, but what's there is a very common idiom and what you suggest is unsafe for if/else
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