From 61d4c87c940fea028f08f27addc275b469320fda Mon Sep 17 00:00:00 2001
From: David Mitchell <davem@iabyn.com>
Date: Mon, 18 Feb 2019 09:19:38 +0000
Subject: [PATCH] Perl_my_cxt_init: fix potential race condition
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
(Found by code inspection - I can't reproduce a failure)
the MY_CXT subsystem, which allows per-thread pseudo-static variables,
has a thread race condition.
When a module is first loaded, it is allocated unique index (from
PL_my_cxt_index++) which is assigned to the module's my_cxt_index static
var.
If two threads both try to load an XS module at the same time, its
possible for one thread to set my_cxtp, then a second thread to overwrite
it with a higher value, causing the first thread to use the wrong index
into its local storage.
Signed-off-by: Petr Písař <ppisar@redhat.com>
---
util.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)
diff --git a/util.c b/util.c
index 5b6f4bfd27..ae86a8c4a4 100644
--- a/util.c
+++ b/util.c
@@ -5218,10 +5218,16 @@ Perl_my_cxt_init(pTHX_ int *index, size_t size)
dVAR;
void *p;
PERL_ARGS_ASSERT_MY_CXT_INIT;
+ /* do initial check without locking.
+ * -1: not allocated or another thread currently allocating
+ * other: already allocated by another thread
+ */
if (*index == -1) {
- /* this module hasn't been allocated an index yet */
MUTEX_LOCK(&PL_my_ctx_mutex);
- *index = PL_my_cxt_index++;
+ /*now a stricter check with locking */
+ if (*index == -1)
+ /* this module hasn't been allocated an index yet */
+ *index = PL_my_cxt_index++;
MUTEX_UNLOCK(&PL_my_ctx_mutex);
}
@@ -5278,9 +5284,12 @@ Perl_my_cxt_init(pTHX_ const char *my_cxt_key, size_t size)
index = Perl_my_cxt_index(aTHX_ my_cxt_key);
if (index == -1) {
- /* this module hasn't been allocated an index yet */
MUTEX_LOCK(&PL_my_ctx_mutex);
- index = PL_my_cxt_index++;
+ /*now a stricter check with locking */
+ index = Perl_my_cxt_index(aTHX_ my_cxt_key);
+ if (index == -1)
+ /* this module hasn't been allocated an index yet */
+ index = PL_my_cxt_index++;
MUTEX_UNLOCK(&PL_my_ctx_mutex);
}
--
2.20.1
