Messages in this thread | | | Date | Wed, 2 May 2012 13:10:04 +1000 | Subject | Re: [PATCH] Describe race of direct read and fork for unaligned buffers | From | Nick Piggin <> |
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On 2 May 2012 13:04, Hugh Dickins <hughd@google.com> wrote: > On Wed, 2 May 2012, Nick Piggin wrote: >> On 2 May 2012 03:56, Michael Kerrisk (man-pages) <mtk.manpages@gmail.com> wrote: >> > >> > In the light of all of the comments, can someone revise the man-pages >> > patch that Jan sent? >> >> This does not quite describe the entire situation, but something understandable >> to developers: >> >> O_DIRECT IOs should never be run concurrently with fork(2) system call, >> when the memory buffer is anonymous memory, or comes from mmap(2) >> with MAP_PRIVATE. >> >> Any such IOs, whether submitted with asynchronous IO interface or from >> another thread in the process, should be quiesced before fork(2) is called. >> Failure to do so can result in data corruption and undefined behavior in >> parent and child processes. >> >> This restriction does not apply when the memory buffer for the O_DIRECT >> IOs comes from mmap(2) with MAP_SHARED or from shmat(2). > > Nor does this restriction apply when the memory buffer has been advised > as MADV_DONTFORK with madvise(2), ensuring that it will not be available > to the child after fork(2).
Yes of course, I forgot that was exported too.
> >> >> >> >> Is that on the right track? I feel it might be necessary to describe this >> allowance for MAP_SHARED, because some databases may be doing >> such things, and anyway it gives apps a potential way to make this work >> if concurrent fork + DIO is very important. > > Looks good, but we do need a reference to MADV_DONTFORK, perhaps as above.
Yep, thanks Hugh.
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