1. in container_free, set c->privlock to NULL before calling
sem_destroy, to prevent a window where another thread could call
sem_wait(c->privlock) while c->privlock is not NULL but is already
destroyed.
2. in container_get, check for numthreads < 0 before calling lxclock.
Once numthreads is 0, it never goes back up.
Following is a comment added to lxccontainer.c:
/*
* Consider the following case:
freer | racing get()er
==================================================================
lxc_container_put() | lxc_container_get()
\ lxclock(c->privlock) | c->numthreads < 1? (no)
\ c->numthreads = 0 | \ lxclock(c->privlock) -> waits
\ lxcunlock() | \
\ lxc_container_free() | \ lxclock() returns
| \ c->numthreads < 1 -> return 0
\ \ (free stuff) |
\ \ sem_destroy(privlock) |
* When the get()er checks numthreads the first time, one of the following
* is true:
* 1. freer has set numthreads = 0. get() returns 0
* 2. freer is between lxclock and setting numthreads to 0. get()er will
* sem_wait on privlock, get lxclock after freer() drops it, then see
* numthreads is 0 and exit without touching lxclock again..
* 3. freer has not yet locked privlock. If get()er runs first, then put()er
* will see --numthreads = 1 and not call lxc_container_free().
*/
Signed-off-by: Serge Hallyn <serge.hal...@ubuntu.com>
---
src/lxc/lxccontainer.c | 33 +++++++++++++++++++++++++++++++--
1 file changed, 31 insertions(+), 2 deletions(-)
diff --git a/src/lxc/lxccontainer.c b/src/lxc/lxccontainer.c
index a4376b4..e09b3fb 100644
--- a/src/lxc/lxccontainer.c
+++ b/src/lxc/lxccontainer.c
@@ -72,9 +72,10 @@ static void lxc_container_free(struct lxc_container *c)
c->slock = NULL;
}
if (c->privlock) {
- sem_destroy(c->privlock);
- free(c->privlock);
+ sem_t *l = c->privlock;
c->privlock = NULL;
+ sem_destroy(l);
+ free(l);
}
if (c->name) {
free(c->name);
@@ -91,11 +92,39 @@ static void lxc_container_free(struct lxc_container *c)
free(c);
}
+/*
+ * Consider the following case:
+freer | racing get()er
+==================================================================
+lxc_container_put() | lxc_container_get()
+\ lxclock(c->privlock) | c->numthreads < 1? (no)
+\ c->numthreads = 0 | \ lxclock(c->privlock) -> waits
+\ lxcunlock() | \
+\ lxc_container_free() | \ lxclock() returns
+ | \ c->numthreads < 1 -> return 0
+\ \ (free stuff) |
+\ \ sem_destroy(privlock) |
+
+ * When the get()er checks numthreads the first time, one of the following
+ * is true:
+ * 1. freer has set numthreads = 0. get() returns 0
+ * 2. freer is between lxclock and setting numthreads to 0. get()er will
+ * sem_wait on privlock, get lxclock after freer() drops it, then see
+ * numthreads is 0 and exit without touching lxclock again..
+ * 3. freer has not yet locked privlock. If get()er runs first, then put()er
+ * will see --numthreads = 1 and not call lxc_container_free().
+*/
+
int lxc_container_get(struct lxc_container *c)
{
if (!c)
return 0;
+ // if someone else has already started freeing the container, don't
+ // try to take the lock, which may be invalid
+ if (c->numthreads < 1)
+ return 0;
+
if (lxclock(c->privlock, 0))
return 0;
if (c->numthreads < 1) {
--
1.8.1.2
------------------------------------------------------------------------------
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