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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 10 Nov 2017, Boris Ostrovsky wrote:
> On 11/10/2017 06:57 PM, Stefano Stabellini wrote:
> > 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:
> > 
> 
> 
> I think you want to say
> 
>       while(mutex_locked(&map->active.in_mutex.owner) ||
>             mutex_locked(&map->active.out_mutex.owner))
>               cpu_relax();
> 
> since owner being NULL is not a documented value of a free mutex.
> 

You are right, and the function name is "mutex_is_locked", so it would
be:


        while(mutex_is_locked(&map->active.in_mutex.owner) ||
              mutex_is_locked(&map->active.out_mutex.owner))
                cpu_relax();


> > 
> >             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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