linux-kernel - Re: [PATCH v3] devres: Explicitly align datai[] to 64-bit

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Date:   Mon, 9 Jul 2018 16:29:58 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "greg@...ah.com" <greg@...ah.com>,
        "will.deacon@....com" <will.deacon@....com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "David.Laight@...LAB.COM" <David.Laight@...LAB.COM>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "geert@...ux-m68k.org" <geert@...ux-m68k.org>
Subject: Re: [PATCH v3] devres: Explicitly align datai[] to 64-bit

On Mon, Jul 09, 2018 at 04:49:25PM +0200, Peter Zijlstra wrote:
> On Mon, Jul 09, 2018 at 02:33:26PM +0000, Alexey Brodkin wrote:
> > > In fact, since alloc_dr() uses kmalloc() to allocate the entire thing,
> > > it is impossible to guarantee a larger alignment than kmalloc does.
> > 
> > Well but 4-bytes [which is critical for atomic64_t] should be much less
> > than a sane cache line length so above should work.
> 
> AFAICT ARCH_KMALLOC_MINALIGN ends up being 4 on x86_32 (it doesn't
> define ARCH_DMA_MINALIGN and doesn't seem to otherwise override the
> thing).

Shouldn't that be 8? AFAICT, __alignof__(unsigned long long) is 8 on
x86_32:

----
[mark@...rids:~]% cat test.c     
#include <stdio.h>

#define PRINT_TYPE_INFO(t) \
        printf("%10s %5d %5d\n", #t, sizeof(t), __alignof__(t))

int main(int argc, char *argv[])
{
        printf("%10s %5s %5s\n", "TYPE", "SIZE", "ALIGN");
        PRINT_TYPE_INFO(int);
        PRINT_TYPE_INFO(long);
        PRINT_TYPE_INFO(long long);

        return 0;
}
[mark@...rids:~]% gcc -m32 test.c -o test
[mark@...rids:~]% ./test 
      TYPE  SIZE ALIGN
       int     4     4
      long     4     4
 long long     8     8
----

Mark.