linux-kernel - RE: [PATCH V2] MIPS: implement smp_cond_load

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Message-ID: <d8a11208170d4fb09564413356ff940c@AcuMS.aculab.com>
Date:   Wed, 11 Jul 2018 11:46:21 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>
CC:     Jiaxun Yang <jiaxun.yang@...goat.com>,
        "linux-mips@...ux-mips.org" <linux-mips@...ux-mips.org>,
        陈华才 <chenhc@...ote.com>,
        Paul Burton <paul.burton@...s.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        "James Hogan" <jhogan@...nel.org>,
        Fuxin Zhang <zhangfx@...ote.com>,
        wuzhangjin <wuzhangjin@...il.com>,
        stable <stable@...r.kernel.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Akira Yokosawa <akiyks@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH V2] MIPS: implement smp_cond_load_acquire() for Loongson-3

From: Peter Zijlstra
> Sent: 11 July 2018 12:10
..
> Adding SYNC to WRITE_ONCE()/atomic* will hurt performance lots and will
> ultimately not guarantee anything more; and as Will said, keep you
> chasing dragons where people forgot to use WRITE_ONCE() where they maybe
> should've.

Also WRITE_ONCE() is there to solve an entirely different problem.
If you have a function that does:
	<lots of code without any function calls>
	foo->bar = 1;
the compiler is allowed to write other (unrelated) values
to foo->bar in the generated code for <lots of code>.

A long time ago I used a compiler that managed to convert:
	if (foo->bar == 2)
		foo->bar = 3;
into:
	if (foo->bar == 2) {
		foo->bar = 0;
		foo->bar = 3;
	}
When an interrupt read the value 0 a lot of linked list got screwed up.
WRITE_ONCE() is there to ensure that doesn't happen.
(In my case 'foo' was a 2-bit wide bitfield, and I suspect you
can't use WRITE_ONCE() on bitfields.)

	David