Re: missing lock in percpu_rwlock? (Was: Re: New Defects reported by Coverity Scan for XenProject)

[Ian Campbell <ian.campbell@citrix.com>]

On Wed, 2016-02-03 at 10:45 +0000, Ian Campbell wrote:
> On Tue, 2016-02-02 at 20:23 -0800, scan-admin@coverity.com wrote:
> > * CID 1351223:  Concurrent data access violations  (MISSING_LOCK)
> > /xen/include/xen/spinlock.h: 362 in _percpu_write_unlock()
> 
> Coverity seems to think this is new in 41b0aa569adb..9937763265d,
> presumably due to 
> 
> commit f9dd43dddc0a31a4343a58072935c1b5c0cbbee
> Author: Malcolm Crossley <malcolm.crossley@citrix.com>
> Date:   Fri Jan 22 16:04:41 2016 +0100
> 
>     rwlock: add per-cpu reader-writer lock infrastructure

It also reports this one, but I suppose this is a false +ve given the name
of the function.

(Also note "simulatenously" should be "simultaneously")

    ** CID 1351220:  Program hangs  (LOCK)
    /xen/include/xen/spinlock.h: 310 in _percpu_read_lock()


    ________________________________________________________________________________________________________
    *** CID 1351220:  Program hangs  (LOCK)
    /xen/include/xen/spinlock.h: 310 in _percpu_read_lock()
    304          * Detect using a second percpu_rwlock_t simulatenously and fallback
    305          * to standard read_lock.
    306          */
    307         if ( unlikely(this_cpu_ptr(per_cpudata) != NULL ) )
    308         {
    309             read_lock(&percpu_rwlock->rwlock);
    >>>     CID 1351220:  Program hangs  (LOCK)
    >>>     Returning without unlocking "percpu_rwlock->rwlock".
    310             return;
    311         }
    312     
    313         /* Indicate this cpu is reading. */
    314         this_cpu_ptr(per_cpudata) = percpu_rwlock;
    315         smp_mb();


> 
> > _______________________________________________________________________
> > __
> > _______________________________
> > *** CID 1351223:  Concurrent data access violations  (MISSING_LOCK)
> > /xen/include/xen/spinlock.h: 362 in _percpu_write_unlock()
> > 356                     percpu_rwlock_t *percpu_rwlock)
> > 357     {
> > 358         /* Validate the correct per_cpudata variable has been
> > provided. */
> > 359         _percpu_rwlock_owner_check(per_cpudata, percpu_rwlock);
> > 360     
> > 361         ASSERT(percpu_rwlock->writer_activating);
> > > > >      CID 1351223:  Concurrent data access violations 
> > > > > (MISSING_LOCK)
> > > > >      Accessing "percpu_rwlock->writer_activating" without holding
> > > > > lock
> > "percpu_rwlock.rwlock". Elsewhere, "percpu_rwlock.writer_activating" is
> > accessed with "percpu_rwlock.rwlock" held 1 out of 2 times (1 of these
> > accesses strongly imply that it is necessary).
> > 362         percpu_rwlock->writer_activating = 0;
> > 363         write_unlock(&percpu_rwlock->rwlock);
> > 364     }
> > 365     
> > 366     #define percpu_rw_is_write_locked(l)        
> > _rw_is_write_locked(&((l)->rwlock))

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