On Mon, Jul 25, 2016 at 05:43:30AM +0200, Emmanuel Dreyfus wrote:
> Vijay Bellur <vbellur@xxxxxxxxxx> wrote:
>
> > Do you have any concrete examples of problems encountered due to the
> > same directory stream being invoked from multiple threads?
>
> I am not sure this scenario can happen, but what we had were directory
> offsets reused among different DIR * opened on the same directory. This
> works on Linux but is a standard violation, as directory offsets are
> supposed to be valid only for a given DIR *.
>
> It broke NetBSD regression enough that I added a test against it in
> xlator/storage/posix/src:posix.c
>
> seekdir (dir, off);
> #ifndef GF_LINUX_HOST_OS
> if ((u_long)telldir(dir) != off && off != pfd->dir_eof)
> {
> gf_msg (THIS->name, GF_LOG_ERROR, EINVAL,
> P_MSG_DIR_OPERATION_FAILED,
> "seekdir(0x%llx) failed on dir=%p: "
> "Invalid argument (offset reused from "
> "another DIR * structure?)", off, dir);
> errno = EINVAL;
> count = -1;
> goto out;
> }
> #endif /* GF_LINUX_HOST_OS */
>
>
>
> About standards and portability, here is the relevant part in Linux man
> page:
> > In the current POSIX.1 specification (POSIX.1-2008), readdir(3) is not
> > required to be thread-safe. However, in modern implementations
> > (including the glibc implementation), concurrent calls to readdir(3)
> > that specify different directory streams are thread-safe.
> >
> > It is expected that a future version of POSIX.1 will make readdir_r()
> > obsolete, and require that readdir() be thread-safe when concurrently
> > employed on different directory streams.
>
> This means linux recommands using readir(), but such practice is likely
> to break on other systems, since standards do not currently requires it
> to be thread-safe. We can go the readdir() way, but pleas add locks.
>
> Alternatively we can use #ifdef to use alternate code on Linux (readdir)
> and others (readdir_r)
This 2nd approach is indeed what Kaleb implemented in
http://review.gluster.org/14838 and has mentioned in an other reply in
the thread.
HTH,
Niels
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