[Gluster-devel] How does GD_SYNCOP work?

Krishnan Parthasarathi kparthas at redhat.com
Mon Sep 15 07:32:40 UTC 2014


Pranith works on glustershd, CC'ing him.


----- Original Message -----
> Emmanuel Dreyfus <manu at netbsd.org> wrote:
> > Here is the problem: once readdir() has reached the end of the
> > directory, on Linux, telldir() will report the last entry's offset,
> > while on NetBSD, it will report an invalid offset (it is in fact the
> > offset of the next entry beyond the last one, which does not exist).
> But that difference did not explain why NetBSD was looping. I discovered
> why.
> Between each index_fill_readdir() invocation, we have a closedir()/opendir()
> invocation. Then index_fill_readdir()  calls seekdir() with a pointer
> obtained from telldir() on the previously open/closed DIR *. Offsets
> returned by telldir() are only valid for a DIR * lifetime [1]. Such rule
> makes sense: If the directory content changed, we are likely to return
> garbage.
> Now if the directory content did not change and we have readen everything,
> here is what happens:
> On Linux, seekdir() works with the offset obtained from previous DIR * (it
> does not have to according to the standards), and goes to the last entry. It
> exits gracefuly returning EOF.
> On NetBSD, seekdir() is given the offset from previous DIR * beyond the last
> entry. It fails and is nilpotent. Subsequent readdir_r() will operate from
> the beginning of the directory, and we never get EOF. Here is our infinite
> loop.
> The correct fix is:
> 1) either to keep the directory open between index_fill_readdir()
> invocations, but since that means preserving an open directory accross
> different syncop, I am not sure it is a good idea.
> 2) do not reuse the offset from last attempt. That means if the buffer get
> filled, resize it as bigger and retry, until the data fits. This is bad
> performance wise, but it seems the only safe way to me.
> Opinions?
> [1] http://pubs.opengroup.org/onlinepubs/009695399/functions/seekdir.html
> --
> Emmanuel Dreyfus
> http://hcpnet.free.fr/pubz
> manu at netbsd.org

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