Re: [PATCH] change thread-unsafe readdir to thread-safe readdir_r calls

[Jason Gunthorpe <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org>]

On Wed, Jul 07, 2010 at 11:37:01AM -0700, Steven Dake wrote:
> From: Steven Dake <sdake-AqqL/iNfm4CbU/LNMs03Rw@public.gmane.org>
> 
> The readdir POSIX api is not thread safe.  This presents problems in
> multithreaded programs that use the libibverbs APIs.  This patch has been
> tested with a libibverbs application (http://www.corosync.org) on
> Mellanox MT26428 cards.

Well, that isn't true on glibc.

The buffer returned by readdir() is stored in the DIR * and is
allocated during opendir().
 
The only time you hit thread safety issues is if multiple threads
attempt to use the same DIR at once - glibc has internal locking to
protect the DIR, but the buffer would be re-used.

Since libibverbs doesn't pass the DIR * between threads this usage is
safe.

It isn't entirely clear to me if POSIX definition of readdir as
non-rentrant applies to the general case or to the specific case I
discuss above. Certainly, any implementation can do as glibc does and
allocate the buffer in the DIR *.

But, you surely could not have seen a bug on Linux that was caused by
this - so why was this patch prepared?

BTW, POSIX also says that NAME_MAX is optional, so you have traded one
unlikely portability problem for another unlikely portability problem :)

Jason
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