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

[Chris Patterson <cjp256@xxxxxxxxx>]

On Tue, May 31, 2016 at 1:53 PM, Wei Liu <wei.liu2@xxxxxxxxxx> wrote:
> 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.
>

I'm far from the expert here, but it would appear that both NetBSD's
and FreeBSD's libc readdir()/readdir_r() implementations are
consistent in their locking mechanisms [1,2].  As such, I would expect
readdir() to be equally as safe as readdir_r() for their users.  As
you noted, there does not appear to be any documented distinction
within their man pages [3,4] with regards to thread safety and it
seems reasonable that they would have documented these limitations, if
present.

[1] FreeBSD: 
https://svnweb.freebsd.org/base/head/lib/libc/gen/readdir.c?view=markup#l98
[2] NetBSD: 
http://cvsweb.netbsd.org/bsdweb.cgi/src/lib/libc/gen/readdir.c?rev=1.25.6.1&content-type=text/x-cvsweb-markup
[3] FreeBSD readdir manpage:
https://www.freebsd.org/cgi/man.cgi?query=readdir&section=3
[4] NetBSD readdir manpage: http://netbsd.gw.com/cgi-bin/man-cgi?readdir

Are there other known supported libc implementations?

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