[RFC PATCH] xs: use system's default stack size for xs_watch's reader thread

[Chris Patterson <cjp256@gmail.com>]

From: Chris Patterson <pattersonc@ainfosec.com>

xs_watch() creates a thread to listen to xenstore events.  Currently, the
thread is created with the greater of 16K or PTHREAD_MIN_SIZE.

There have been several bug reports and workarounds related to the issue
where xs_watch() fails because its attempt to create the reader thread with
pthread_create() fails. This is due to insufficient stack space size
given the requirements for thread-local storage usage in the applications
and libraries that are linked against libxenstore.  [1,2,3,4].

Specifying the stack size appears to have been added to reduce memory
footprint (1d00c73b983b09fbee4d9dc0f58f6663c361c345).

This has already been bumped up once to the greater of 16k and
PTHREAD_STACK_MIN (da6a0e86d6a079102abdd0858a19f1e1fae584fc).

This patch reverts to sticking with the system's default stack size and
removes the code used to set the thread's stack size.

Of course, the alternative is to bump it to another arbitrary value, but the
requirements may change depending on the application and its libraries that
are linking against xenstore.

[1] https://lists.nongnu.org/archive/html/qemu-devel/2016-07/msg03341.html
[2] https://lists.xenproject.org/archives/html/xen-users/2016-07/msg00012.html
[3] https://lists.xenproject.org/archives/html/xen-users/2016-07/msg00085.html
[4] https://bugzilla.redhat.com/show_bug.cgi?id=1350264

Signed-off-by: Chris Patterson <pattersonc@ainfosec.com>
---
 tools/xenstore/xs.c | 10 ----------
 1 file changed, 10 deletions(-)

diff --git a/tools/xenstore/xs.c b/tools/xenstore/xs.c
index d1e01ba..c62b120 100644
--- a/tools/xenstore/xs.c
+++ b/tools/xenstore/xs.c
@@ -723,11 +723,6 @@ bool xs_watch(struct xs_handle *h, const char *path, const char *token)
 	struct iovec iov[2];
 
 #ifdef USE_PTHREAD
-#define DEFAULT_THREAD_STACKSIZE (16 * 1024)
-#define READ_THREAD_STACKSIZE 					\
-	((DEFAULT_THREAD_STACKSIZE < PTHREAD_STACK_MIN) ? 	\
-	PTHREAD_STACK_MIN : DEFAULT_THREAD_STACKSIZE)
-
 	/* We dynamically create a reader thread on demand. */
 	mutex_lock(&h->request_mutex);
 	if (!h->read_thr_exists) {
@@ -738,11 +733,6 @@ bool xs_watch(struct xs_handle *h, const char *path, const char *token)
 			mutex_unlock(&h->request_mutex);
 			return false;
 		}
-		if (pthread_attr_setstacksize(&attr, READ_THREAD_STACKSIZE) != 0) {
-			pthread_attr_destroy(&attr);
-			mutex_unlock(&h->request_mutex);
-			return false;
-		}
 
 		sigfillset(&set);
 		pthread_sigmask(SIG_SETMASK, &set, &old_set);
-- 
2.1.4


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