Re: [PATCH] DMA: PL330: Fix racy mutex unlock

[Javi Merino]

On Wed 13 Jun 2012 16:13:05 BST, Jassi Brar wrote:
> On 13 June 2012 19:37, Javi Merino<javi.merino@xxxxxxx>  wrote:
> > pl330_update() stores a pointer to the thrd->req that finished, which
> > contains a pointer to the corresponding pl330_req.  This is done with
> > the pl330_lock held.  Then, it iterates through the req_done list,
> > calling the callback for each of the requests that are done.  The
> > problem is that the driver releases the lock before calling the
> > callback for each of the callbacks.  pl330_submit_req() running in
> > another processor can then acquire the lock and insert another request
> > in one of the thrd->req that hasn't been processed yet, replacing the
> > pointer to pl330_req there.  When the callback returns in
> > pl330_update() and the next rqdone is popped from the list, it
> > dereferences the pl330_req pointer to the just scheduled pl330_req,
> > instead of the one that has finished, calling pl330 with the wrong r.
> >
> > This patch fixes this by storing the pointer to pl330_req directly in
> > the list.
> .....
> > @@ -1683,7 +1683,7 @@ static void pl330_dotask(unsigned long data)
> >   /* Returns 1 if state was updated, 0 otherwise */
> >   static int pl330_update(const struct pl330_info *pi)
> >   {
> > -       struct _pl330_req *rqdone;
> > +       struct pl330_req *rqdone, *tmp;
> >         struct pl330_dmac *pl330;
> >         unsigned long flags;
> >         void __iomem *regs;
> > @@ -1750,7 +1750,10 @@ static int pl330_update(const struct pl330_info *pi)
> >                         if (active == -1) /* Aborted */
> >                                 continue;
> >
> > -                       rqdone =&thrd->req[active];
> > +                       /* Detach the req */
> > +                       rqdone = thrd->req[active].r;
> > +                       thrd->req[active].r = NULL;
> > +
> Doesn't this movement of "Detach the req" chunk effectively remain the
> same? Since that was already protected by the same lock. I thought I
> deliberately took care of that already.

Yes, but you release the lock before calling the first callback, so the 
subsequent dereferences of rqdone->r are not protected by the lock.

> Do you see some real problem fixed by this patch? Info about that
> could help me better understand if I missed something here.

Ok, the description sucks.  Let me try to describe it with the scenario 
that failed:

Core 0:
- Two DMA transactions finish, in channels 0 and 1.
- pl330_update() is called, the "Event-Interrupt Status Register" (ES)
  is 0x3.
- In the "for (ev = 0;..." loop
  + two pointers are stored in pl330->req_done:
    pl330->channels[0]->req[0] and pl330->channels[1]->req[0]
- In the "while (!list_empty.." loop,
  + r = pl330->channels[0]->req[0]->r
  + Release the pl330_lock and call _callback()

Core 1:
- pl330_submit_req() for channel 1
- Grab the pl330_lock
- Hook a request in pl330->channels[1]->req[0]->r
- Release the pl330_lock

Core 0:
- _callback() returns
- Acquire the pl330_lock again
- second iteration of "while (!list_empty.." loop,
  + r = pl330->channels[1]->req[0]->r , but you get the r that has just
    been scheduled, not the one that finished.

Hope it's now clearer,
Javi

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