linux-kernel - Re: single copy atomicity for double load/stores on 32-bit systems

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Message-Id: <20190603201324.GN28207@linux.ibm.com>
Date:   Mon, 3 Jun 2019 13:13:24 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <Will.Deacon@....com>,
        arcml <linux-snps-arc@...ts.infradead.org>,
        lkml <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: single copy atomicity for double load/stores on 32-bit systems

On Mon, Jun 03, 2019 at 06:08:35PM +0000, Vineet Gupta wrote:
> On 5/31/19 1:21 AM, Peter Zijlstra wrote:
> >> I'm not sure how to interpret "natural alignment" for the case of double
> >> load/stores on 32-bit systems where the hardware and ABI allow for 4 byte
> >> alignment (ARCv2 LDD/STD, ARM LDRD/STRD ....)
> > Natural alignment: !((uintptr_t)ptr % sizeof(*ptr))
> >
> > For any u64 type, that would give 8 byte alignment. the problem
> > otherwise being that your data spans two lines/pages etc..
> 
> Sure, but as Paul said, if the software doesn't expect them to be atomic by
> default, they could span 2 hardware lines to keep the implementation simpler/sane.

I could imagine 8-byte types being only four-byte aligned on 32-bit systems,
but it would be quite a surprise on 64-bit systems.

							Thanx, Paul