Re: [RESEND PATCH] hw/dma: fix crash caused by race condition

[David Hildenbrand]

On 29.06.22 10:31, Tong Zhang wrote:
> 
> 
> On Wed, Jun 29, 2022 at 12:29 AM David Hildenbrand <david@redhat.com
> <mailto:david@redhat.com>> wrote:
> 
>     On 06.05.22 18:31, Tong Zhang wrote:
>     > assert(dbs->acb) is meant to check the return value of io_func per
>     > documented in commit 6bee44ea34 ("dma: the passed io_func does not
>     > return NULL"). However, there is a chance that after calling
>     > aio_context_release(dbs->ctx); the dma_blk_cb function is called
>     before
>     > the assertion and dbs->acb is set to NULL again at line 121. Thus when
>     > we run assert at line 181 it will fail.
>     >
>     >   softmmu/dma-helpers.c:181: dma_blk_cb: Assertion `dbs->acb' failed.
>     >
>     > Reported-by: Francisco Londono <f.londono@samsung.com
>     <mailto:f.londono@samsung.com>>
>     > Signed-off-by: Tong Zhang <t.zhang2@samsung.com
>     <mailto:t.zhang2@samsung.com>>
>     > ---
>     >  softmmu/dma-helpers.c | 2 +-
>     >  1 file changed, 1 insertion(+), 1 deletion(-)
>     >
>     > diff --git a/softmmu/dma-helpers.c b/softmmu/dma-helpers.c
>     > index 7820fec54c..cb81017928 100644
>     > --- a/softmmu/dma-helpers.c
>     > +++ b/softmmu/dma-helpers.c
>     > @@ -177,8 +177,8 @@ static void dma_blk_cb(void *opaque, int ret)
>     >      aio_context_acquire(dbs->ctx);
>     >      dbs->acb = dbs->io_func(dbs->offset, &dbs->iov,
>     >                              dma_blk_cb, dbs, dbs->io_func_opaque);
>     > -    aio_context_release(dbs->ctx);
>     >      assert(dbs->acb);
>     > +    aio_context_release(dbs->ctx);
>     >  }
>     > 
>     >  static void dma_aio_cancel(BlockAIOCB *acb)
> 
>     Please don't resend patches if the previous submission came to the
>     conclusion that it's unclear how this should help.
> 
>     
> https://lkml.kernel.org/r/CAJSP0QW396RY_g8LS1mncDZcOv5GamURy+xv+s8zMcdq03OOMA@mail.gmail.com
>     
> <https://lkml.kernel.org/r/CAJSP0QW396RY_g8LS1mncDZcOv5GamURy+xv+s8zMcdq03OOMA@mail.gmail.com>
> 
> 
>     I *still* don't understand the interaction between the lock and the
>     assertion and so far nobody was able to clarify.
> 
>     -- 
>     Thanks,
> 
>     David / dhildenb
> 
> hello
> 
> This message is sent way before the discussion 

Oh, I'm sorry. I was mislead by the reply from Laurent :)

BTW, do we now have an understanding why that patch helps and if it
applies to upstream?

-- 
Thanks,

David / dhildenb