Re: [Xen-devel] [PATCH 1/2] libfsimage: replace deprecated readdir_r() with readdir()

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, May 31, 2016 at 11:43:13AM -0400, Chris Patterson wrote:
> On Tue, May 31, 2016 at 6:42 AM, George Dunlap <george.dunlap@xxxxxxxxxx> 
> wrote:
> > On Mon, May 30, 2016 at 3:32 AM, Chris Patterson <cjp256@xxxxxxxxx> wrote:
> >> From: Chris Patterson <pattersonc@xxxxxxxxxxxx>
> >>
> >> Replace the usage of readdir_r() with readdir() to address
> >> a compilation error due to the deprecation of readdir_r.
> >>
> >> glibc has deprecated this for their next release (2.24):
> >> https://sourceware.org/bugzilla/show_bug.cgi?id=19056
> >>
> >> Signed-off-by: Chris Patterson <pattersonc@xxxxxxxxxxxx>
> >
> > Thanks for the patch, Chris.  A bit more background would have been
> > helpful -- I did some searching and found a description[1] which says:
> >
> 
> Thank you. I should have added these details in the commit message or cover.
> 
> > 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.  Therefore, the use of readdir_r() is generally
> > unnecessary in multithreaded programs.  In cases where multiple
> > threads must read from the same directory stream, using readdir(3)
> > with external synchronization is still preferable to the use of
> > readdir_r(), for the reasons given in the points above.
> >
> > The use of the specific directory stream is single-threaded, so for
> > glibc, it looks like using readdir() will be safe.  But libxl needs to
> > be able to build on a number of libc's that are not glibc and still be
> > thread-safe.  So we need to either 1) verify that readdir() is thread
> > safe on all libc's against which we may compile, or 2) add some
> 
> Is there a list of supported libc libraries?  I can look into it and
> provide more definitive answers if there are.
> 

We at least care about FreeBSD and NetBSD. Sadly their manuaks don't
provide statement regarding thread safety for readdir and readdir_r.
It's better to err on the safe side.

Wei.

> > #ifdef-ery to switch to readdir_r() on systems unless we know it's
> > thread-safe.
> >
> > I'm less familiar with best practices for this kind of thing -- Ian,
> > do you have an idea?
> >
> > Thanks again for raising this, Chris.
> >
> >  -George
> >
> > [1] http://man7.org/linux/man-pages/man3/readdir_r.3.html
> >

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