linux-kernel - RE: [PATCH v2 2/6] bitops: always define asm-generic non-atomic bitops

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Date:   Fri, 10 Jun 2022 16:02:03 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Lobakin, Alexandr" <alexandr.lobakin@...el.com>
CC:     Arnd Bergmann <arnd@...db.de>, Yury Norov <yury.norov@...il.com>,
        "Mark Rutland" <mark.rutland@....com>,
        Matt Turner <mattst88@...il.com>,
        Brian Cain <bcain@...cinc.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        "Yoshinori Sato" <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Kees Cook <keescook@...omium.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Marco Elver <elver@...gle.com>, Borislav Petkov <bp@...e.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-alpha@...r.kernel.org" <linux-alpha@...r.kernel.org>,
        "linux-hexagon@...r.kernel.org" <linux-hexagon@...r.kernel.org>,
        "linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
        "linux-m68k@...ts.linux-m68k.org" <linux-m68k@...ts.linux-m68k.org>,
        "linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 2/6] bitops: always define asm-generic non-atomic
 bitops

> > +/**
> > + * generic_test_bit - Determine whether a bit is set
> > + * @nr: bit number to test
> > + * @addr: Address to start counting from
> > + */
>
> Shouldn't we add in this or in separate patch a big NOTE to explain that this
> is actually atomic and must be kept as a such?

"atomic" isn't really the right word. The volatile access makes sure that the
compiler does the test at the point that the source code asked, and doesn't
move it before/after other operations.

But there is no such thing as an atomic test_bit() operation:

	if (test_bit(5, addr)) {
		/* some other CPU nukes bit 5 */

		/* I know it was set when I looked, but now, could be anything */

		...
	}

-Tony