Re: [PATCH] Describe race of direct read and fork for unaligned buffers

[Michael Kerrisk (man-pages)]

On Wed, May 2, 2012 at 4:15 AM, KOSAKI Motohiro
<kosaki.motohiro@xxxxxxxxx> wrote:
>> +suffices. However, if the user buffer is not page aligned and direct read
>
> One more thing. direct write also makes data corruption. Think
> following scenario,

In the light of all of the comments, can someone revise the man-pages
patch that Jan sent?

Thanks,

Michael


> 1) P1-T1 uses DIO write (and starting dma)
> 2) P1-T2 call fork() and makes P2
> 3) P1-T3 write to the dio target page. and then, cow break occur and
> original dio target
>    pages is now owned by P2.
> 4) P2 write the dio target page. It now does NOT make cow break. and
> now we break
>    dio target page data.
> 5) DMA transfer write invalid data to disk.
>
> The detail is described in your refer URLs.
>
>
>> +runs in parallel with a
>> +.BR fork (2)
>> +of the reader process, it may happen that the read data is split between
>> +pages owned by the original process and its child. Thus effectively read
>> +data is corrupted.
>>  .LP
>>  The
>>  .B O_DIRECT



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface"; http://man7.org/tlpi/
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