linux-kernel - Re: [PATCH v5 2/2] x86/asm/bitops: __ffs,ffz: use __builtin

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Date:   Wed, 24 Aug 2022 10:43:37 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Vincent MAILHOL <mailhol.vincent@...adoo.fr>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, David Howells <dhowells@...hat.com>,
        Jan Beulich <JBeulich@...e.com>,
        Christophe Jaillet <christophe.jaillet@...adoo.fr>,
        Joe Perches <joe@...ches.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>
Subject: Re: [PATCH v5 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to
 evaluate constant expressions

On Wed, Aug 24, 2022 at 05:31:20AM +0900, Vincent MAILHOL wrote:
> If the fact that __ffs(0) is undefined is a concern,

So what is of concern is I'm looking at those *ffs things and they look
like a real mess:

 * Undefined if no bit exists, so code should check against 0 first.
 */
static __always_inline unsigned long __ffs(unsigned long word)
{
        asm("rep; bsf %1,%0"

and that's TZCNT.

And nowhere in TZCNT's description does it talk about undefined behavior
- it is all defined.

So I have no clue what that comment is supposed to mean?

Then:

 * ffs - find first set bit in word
 * @x: the word to search
 *
 * This is defined the same way as the libc and compiler builtin ffs
 * routines, therefore differs in spirit from the other bitops.
 *
 * ffs(value) returns 0 if value is 0 or the position of the first
 * set bit if value is nonzero. The first (least significant) bit
 * is at position 1.

while

"Built-in Function: int __builtin_ctz (unsigned int x)

    Returns the number of trailing 0-bits in x, starting at the least significant bit position. If x is 0, the result is undefined."

as previously pasted.

So ffs() doesn't have undefined behavior either.

I guess it wants to say, it is undefined in the *respective* libc or
compiler helper implementation. And that should be explained.

> I can add a safety net:

Nah, no need. It seems this "behavior" has been the case a long time so
callers should know better (or have burned themselves properly :)).

> There is an index issue. __ffs() starts at 0 but ffs() starts at one.
> i.e.: __ffs(0x01) is 0 but ffs(0x01) is 1.
> Aside from the zero edge case, ffs(x) equals __ffs(x) + 1. This
> explains why __fss(0) is undefined.

I'd love to drop the undefined thing and start counting at 1 while
keeping the 0 case the special one.

But that ship has sailed a long time ago - look at all the __ffs() and
ffs() callers.

Back to your patch: I think the text should be fixed to say that both
ffs() and __ffs()'s kernel implementation doesn't have undefined results
but since it needs to adhere to the libc variants' API, it treats 0
differently. They surely can handle 0 as input.

I.e., I'd like to see a comment there explaining the whole difference
between ffs() and __ffs() so that people are aware.

Btw, pls do

s/variable___ffs/variable__ffs/g

Two underscores are just fine.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette