linux-kernel - Re: x86-32: clean up rwsem inline asm statements

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Message-ID: <20100114002708.4154.qmail@science.horizon.com>
Date:	13 Jan 2010 19:27:08 -0500
From:	"George Spelvin" <linux@...izon.com>
To:	hpa@...or.com, linux@...izon.com
Cc:	linux-kernel@...r.kernel.org, schwab@...ux-m68k.org,
	torvalds@...ux-foundation.org
Subject: Re: x86-32: clean up rwsem inline asm statements

> There are a number of things that can be done better... for one thing,
> "+m" (sem->count) and "a" (sem) is just bloody wrong.  The right thing
> would be "a" (&sem->count) for proper robustness.

Actually, no.  The "+m" (sem->count) is telling GCC that sem->count is
updated; "a" (&sem->count) does *not* tell it to invalidate cached
copies of sem->count that it may have lying around.

However, we can't just use "+m" (sem->count) because GCC has a poor
grasp on the concept of atomic operations; as far as it is concerned,
it is exactly equivalent to copying the value into a stack slot, do the
operation there, and copy it back.

(It's much more likely to do that with "g" operand constraints, but it has
been known to point to a stack copy that it's made for some other reason.)

The current situation is, as far as I can remember the previous discussion
on the subject, the simplest way to explain to GCC just what it needs to do.

> There is no real point in being concerned about the type of immediates,
> because the immediate type isn't really used... it shows up as a literal
> in the assembly language.  However, if you're really concerned, the
> right thing to do is to do a cast in C, not playing games with the assembly.

Ah, right, sorry; I remembered having this problem, but it was with
register operands.
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