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SubjectRe: [PATCH] Flush dcache before writing into page to avoid alias
* Russell King - ARM Linux | 2010-01-25 20:00:04 [+0000]:

>On Mon, Jan 25, 2010 at 11:58:14AM -0800, Andrew Morton wrote:
>> On Mon, 25 Jan 2010 21:33:08 +0800 anfei <anfei.zhou@gmail.com> wrote:
>>
>> > Hi Andrew,
>> >
>> > On Thu, Jan 21, 2010 at 01:07:57PM +0800, anfei zhou wrote:
>> > > The cache alias problem will happen if the changes of user shared mapping
>> > > is not flushed before copying, then user and kernel mapping may be mapped
>> > > into two different cache line, it is impossible to guarantee the coherence
>> > > after iov_iter_copy_from_user_atomic. So the right steps should be:
>> > > flush_dcache_page(page);
>> > > kmap_atomic(page);
>> > > write to page;
>> > > kunmap_atomic(page);
>> > > flush_dcache_page(page);
>> > > More precisely, we might create two new APIs flush_dcache_user_page and
>> > > flush_dcache_kern_page to replace the two flush_dcache_page accordingly.
>> > >
>> > > Here is a snippet tested on omap2430 with VIPT cache, and I think it is
>> > > not ARM-specific:
>> > > int val = 0x11111111;
>> > > fd = open("abc", O_RDWR);
>> > > addr = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
>> > > *(addr+0) = 0x44444444;
>> > > tmp = *(addr+0);
>> > > *(addr+1) = 0x77777777;
>> > > write(fd, &val, sizeof(int));
>> > > close(fd);
>> > > The results are not always 0x11111111 0x77777777 at the beginning as expected.
>> > >
>> > Is this a real bug or not necessary to support?
>>
>> Bug. If variable `addr' has type int* then the contents of that file
>> should be 0x11111111 0x77777777. You didn't tell us what the contents
>> were in the incorrect case, but I guess it doesn't matter.
>
>FYI, from a previous email from anfei:
>
>0x44444444 0x77777777

I just wanted to query what the status of this patch is. This patch
seems to fix a real bug which causes a test suite to fail on ARM [0].
The test suite passes on my VIVT ARM with this patch.

[0] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=524003

Sebastian


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