linux-kernel - Re: enabling libgcc for 64-bit divisions, was Re: PROBLEM: XFS on ARM corruption 'Structure needs cl

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Date:	13 Aug 2015 14:18:41 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	linux@...izon.com, torvalds@...ux-foundation.org
Cc:	akpm@...ux.foundation.org, hch@...radead.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	luto@...nel.org
Subject: Re: enabling libgcc for 64-bit divisions, was Re: PROBLEM: XFS on ARM corruption 'Structure needs cl

> I'm not convinced that "64/32->32" is all that generic, though.  If
> the dividend in 64-bit, there's no fundamental type-based guarantee
> that things will fit.

I agree that it's impossible to decide based on the types, but having
that knowledge is extremely common.  Which is why it would be nice
to have a way for the programmer to communicate that knowledge.

> So your case is rather special, and depends (intimately) on knowing
> the actual ranges and how they interact.

Actually, it's the most common case.  Going through "git grep -w do_div",
by far the *majority* of all calls to do_div immediately convert the
result to 32 bits (or unsigned long), with no overflow checking.

Partially that's because I'm cointing static code frequency and there
are a ridiculous number of different PLL drivers, but still.

On x86, the case that msword >= divsor causes a divide exception
(divide ba generalization of divide by zero), so it's tempting
to do the same sort of "assume no trap and fix up in the handler"
trick as <asm/uaccess.h>.

There are only 854 references to do_div in the kernel, so
doing a sweep over all of them is quite practical.

One function that would cover a significant number of use cases
(but not all, damn it) would be

rem = do_mul_div(x, mul,_div)

Which returns x * mul / div, with a 64-bit intermediate.
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