[lxc-devel] [PATCH RFC] Fix up struct lxc_container locking
Seth Arnold
seth.arnold at canonical.com
Thu Apr 11 17:04:40 UTC 2013
On Thu, Apr 11, 2013 at 11:43:31AM -0500, 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.hallyn at ubuntu.com>
Acked-by: Seth Arnold <seth.arnold at canonical.com>
Thanks
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