On 04/11/2013 06:43 PM, Serge Hallyn wrote: > 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>
Acked-by: Stéphane Graber <stgra...@ubuntu.com>
Pushing to staging.
> ---
> 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) {
>
--
Stéphane Graber
Ubuntu developer
http://www.ubuntu.com
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