[Gluster-devel] readdir() harmful in threaded code

[Emmanuel Dreyfus]

Vijay Bellur <vbellur at redhat.com> 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)
-- 
Emmanuel Dreyfus
http://hcpnet.free.fr/pubz
manu at netbsd.org