Re: [Xen-devel] [PATCH v2] libxenstore: prefer using the character device

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Aug 27, 2015 at 09:04:38AM -0500, Jonathan Creekmore wrote:
> With the addition of FMODE_ATOMIC_POS in the Linux 3.14 kernel,
> concurrent blocking file accesses to a single open file descriptor can
> cause a deadlock trying to grab the file position lock. If a watch has
> been set up, causing a read_thread to blocking read on the file
> descriptor, then future writes that would cause the background read to
> complete will block waiting on the file position lock before they can
> execute. This race condition only occurs when libxenstore is accessing
> the xenstore daemon through the /proc/xen/xenbus file and not through
> the unix domain socket, which is the case when the xenstore daemon is
> running as a stub domain or when oxenstored is passed
> --disable-socket. Accessing the daemon from the true character device
> also does not exhibit this problem.
> 
> On Linux, prefer using the character device file over the proc file if
> the character device exists.
> 
> Signed-off-by: Jonathan Creekmore <jonathan.creekmore@xxxxxxxxx>

Acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>

I think this is a candidate for 4.6.

> ---
>  tools/xenstore/xs_lib.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/tools/xenstore/xs_lib.c b/tools/xenstore/xs_lib.c
> index af4f75a..0c7744e 100644
> --- a/tools/xenstore/xs_lib.c
> +++ b/tools/xenstore/xs_lib.c
> @@ -81,6 +81,8 @@ const char *xs_domain_dev(void)
>  #if defined(__RUMPUSER_XEN__) || defined(__RUMPRUN__)
>       return "/dev/xen/xenbus";
>  #elif defined(__linux__)
> +     if (access("/dev/xen/xenbus", F_OK) == 0)
> +             return "/dev/xen/xenbus";
>       return "/proc/xen/xenbus";
>  #elif defined(__NetBSD__)
>       return "/kern/xen/xenbus";
> -- 
> 2.1.4
> 
> 
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