Re: [PATCH] locking/qrwlock: Fix ordering in queued_write_lock_slowpath

[Catalin Marinas]

On Thu, Apr 15, 2021 at 04:28:21PM +0100, Will Deacon wrote:
> On Thu, Apr 15, 2021 at 05:03:58PM +0200, Peter Zijlstra wrote:
> > On Thu, Apr 15, 2021 at 02:25:52PM +0000, Ali Saidi wrote:
> > > While this code is executed with the wait_lock held, a reader can
> > > acquire the lock without holding wait_lock.  The writer side loops
> > > checking the value with the atomic_cond_read_acquire(), but only truly
> > > acquires the lock when the compare-and-exchange is completed
> > > successfully which isn’t ordered. The other atomic operations from this
> > > point are release-ordered and thus reads after the lock acquisition can
> > > be completed before the lock is truly acquired which violates the
> > > guarantees the lock should be making.
[...]
> > > Fixes: b519b56e378ee ("locking/qrwlock: Use atomic_cond_read_acquire() when spinning in qrwloc")
> > > Signed-off-by: Ali Saidi <alisaidi@xxxxxxxxxx>
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > ---
> > >  kernel/locking/qrwlock.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
> > > index 4786dd271b45..10770f6ac4d9 100644
> > > --- a/kernel/locking/qrwlock.c
> > > +++ b/kernel/locking/qrwlock.c
> > > @@ -73,8 +73,8 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
> > >  
> > >  	/* When no more readers or writers, set the locked flag */
> > >  	do {
> > > -		atomic_cond_read_acquire(&lock->cnts, VAL == _QW_WAITING);
> > > -	} while (atomic_cmpxchg_relaxed(&lock->cnts, _QW_WAITING,
> > > +		atomic_cond_read_relaxed(&lock->cnts, VAL == _QW_WAITING);
> > > +	} while (atomic_cmpxchg_acquire(&lock->cnts, _QW_WAITING,
> > >  					_QW_LOCKED) != _QW_WAITING);
> > >  unlock:
> > >  	arch_spin_unlock(&lock->wait_lock);
> > 
> > This doesn't make sense, there is no such thing as a store-acquire. What
> > you're doing here is moving the acquire from one load to the next. A
> > load we know will load the exact same value.
> > 
> > Also see Documentation/atomic_t.txt:
> > 
> >   {}_acquire: the R of the RMW (or atomic_read) is an ACQUIRE
> > 
> > 
> > If anything this code wants to be written like so.
> > 
> > ---
> > 
> > diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
> > index 4786dd271b45..22aeccc363ca 100644
> > --- a/kernel/locking/qrwlock.c
> > +++ b/kernel/locking/qrwlock.c
> > @@ -60,6 +60,8 @@ EXPORT_SYMBOL(queued_read_lock_slowpath);
> >   */
> >  void queued_write_lock_slowpath(struct qrwlock *lock)
> >  {
> > +	u32 cnt;
> > +
> >  	/* Put the writer into the wait queue */
> >  	arch_spin_lock(&lock->wait_lock);
> >  
> > @@ -73,9 +75,8 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
> >  
> >  	/* When no more readers or writers, set the locked flag */
> >  	do {
> > -		atomic_cond_read_acquire(&lock->cnts, VAL == _QW_WAITING);
> > -	} while (atomic_cmpxchg_relaxed(&lock->cnts, _QW_WAITING,
> > -					_QW_LOCKED) != _QW_WAITING);
> > +		cnt = atomic_cond_read_acquire(&lock->cnts, VAL == _QW_WAITING);
> 
> I think the issue is that >here< a concurrent reader in interrupt context
> can take the lock and release it again, but we could speculate reads from
> the critical section up over the later release and up before the control
> dependency here...
> 
> > +	} while (!atomic_try_cmpxchg_relaxed(&lock->cnts, &cnt, _QW_LOCKED));
> 
> ... and then this cmpxchg() will succeed, so our speculated stale reads
> could be used.
> 
> *HOWEVER*
> 
> Speculating a read should be fine in the face of a concurrent _reader_,
> so for this to be an issue it implies that the reader is also doing some
> (atomic?) updates.

There's at least one such case: see chain_epi_lockless() updating
epi->next, called from ep_poll_callback() with a read_lock held. This
races with ep_done_scan() which has the write_lock held.

I think the authors of the above code interpreted the read_lock as
something that multiple threads can own disregarding the _read_ part.

-- 
Catalin