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

Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx> writes:

Wei Liu writes ("Re: [Xen-devel] [PATCH v2] libxenstore: prefer using the character device"):
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.

I confess I still see this as working around a kernel bug. Only this time we are switching from a buggy to non-buggy kernel interface.
Why don't we have the kernel provide only non-buggy interfaces ?

I was just trying to implement what David suggested; CC'ing him on this as well.

> 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";

Also, previously xs_domain_dev was a function which simply returned a static value. I feel vaguely uneasy at putting this kind of autodetection logic here.

Not entirely; the existing code queried an environment variable first
and, only if that was not set, did it return a static value. I added the
autodetection logic to fall back in the case where, for some reason,
/dev/xen/xenbus did not exist. Handling that kind of a fallback at a
higher layer would be a larger refactor to probe for the existence of
the character device first.

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