Re: [Xen-devel] [PATCH] xen/pvcalls: fix potential endless loop in pvcalls-front.c

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 7 Nov 2017, Juergen Gross wrote:
> On 06/11/17 23:17, Stefano Stabellini wrote:
> > mutex_trylock() returns 1 if you take the lock and 0 if not. Assume you
> > take in_mutex on the first try, but you can't take out_mutex. Next times
> > you call mutex_trylock() in_mutex is going to fail. It's an endless
> > loop.
> > 
> > Solve the problem by moving the two mutex_trylock calls to two separate
> > loops.
> > 
> > Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> > Signed-off-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > CC: boris.ostrovsky@xxxxxxxxxx
> > CC: jgross@xxxxxxxx
> > ---
> >  drivers/xen/pvcalls-front.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/xen/pvcalls-front.c b/drivers/xen/pvcalls-front.c
> > index 0c1ec68..047dce7 100644
> > --- a/drivers/xen/pvcalls-front.c
> > +++ b/drivers/xen/pvcalls-front.c
> > @@ -1048,8 +1048,9 @@ int pvcalls_front_release(struct socket *sock)
> >              * is set to NULL -- we only need to wait for the existing
> >              * waiters to return.
> >              */
> > -           while (!mutex_trylock(&map->active.in_mutex) ||
> > -                      !mutex_trylock(&map->active.out_mutex))
> > +           while (!mutex_trylock(&map->active.in_mutex))
> > +                   cpu_relax();
> > +           while (!mutex_trylock(&map->active.out_mutex))
> >                     cpu_relax();
> 
> Any reason you don't just use mutex_lock()?

Hi Juergen, sorry for the late reply.

Yes, you are right. Given the patch, it would be just the same to use
mutex_lock.

This is where I realized that actually we have a problem: no matter if
we use mutex_lock or mutex_trylock, there are no guarantees that we'll
be the last to take the in/out_mutex. Other waiters could be still
outstanding.

We solved the same problem using a refcount in pvcalls_front_remove. In
this case, I was thinking of reusing the mutex internal counter for
efficiency, instead of adding one more refcount.

For using the mutex as a refcount, there is really no need to call
mutex_trylock or mutex_lock. I suggest checking on the mutex counter
directly:


                while (atomic_long_read(&map->active.in_mutex.owner) != 0UL ||
                       atomic_long_read(&map->active.out_mutex.owner) != 0UL)
                        cpu_relax();

Cheers,

Stefano


---

xen/pvcalls: fix potential endless loop in pvcalls-front.c

mutex_trylock() returns 1 if you take the lock and 0 if not. Assume you
take in_mutex on the first try, but you can't take out_mutex. Next time
you call mutex_trylock() in_mutex is going to fail. It's an endless
loop.

Actually, we don't want to use mutex_trylock at all: we don't need to
take the mutex, we only need to wait until the last mutex waiter/holder
releases it.

Instead of calling mutex_trylock or mutex_lock, just check on the mutex
refcount instead.

Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Signed-off-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: boris.ostrovsky@xxxxxxxxxx
CC: jgross@xxxxxxxx

diff --git a/drivers/xen/pvcalls-front.c b/drivers/xen/pvcalls-front.c
index 0c1ec68..9f33cb8 100644
--- a/drivers/xen/pvcalls-front.c
+++ b/drivers/xen/pvcalls-front.c
@@ -1048,8 +1048,8 @@ int pvcalls_front_release(struct socket *sock)
                 * is set to NULL -- we only need to wait for the existing
                 * waiters to return.
                 */
-               while (!mutex_trylock(&map->active.in_mutex) ||
-                          !mutex_trylock(&map->active.out_mutex))
+               while (atomic_long_read(&map->active.in_mutex.owner) != 0UL ||
+                      atomic_long_read(&map->active.out_mutex.owner) != 0UL)
                        cpu_relax();
 
                pvcalls_front_free_map(bedata, map);

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