[PATCH v3] malloc: Fix list_lock/arena lock deadlock [BZ #19182]

[Torvald Riegel]

On Thu, 2015-12-17 at 20:18 +0100, Florian Weimer wrote:
> On 12/17/2015 08:05 PM, Torvald Riegel wrote:
> 
> >> +  (void) mutex_lock (&free_list_lock);
> >> +  detach_arena (replaced_arena);
> >> +  (void) mutex_unlock (&free_list_lock);
> >> +
> >> +  /* Lock this arena.  NB: We may not have exclusive access to this
> >> +     arena anymore because the arena is now accessible from the
> >> +     main_arena.next list and could have been picked by reused_arena
> >> +     in the meantime.  This can only happen for the last arena created
> >> +     (before the arena limit is reached).  This seems unlikely, so we
> >> +     do not protect against this special case.  */
> > 
> > "This seems unlikely" is not sufficient for correctness, IMO.
> 
> Just one quick comment: It's not a correctness issue.  I will clarify
> that the new arena can be attached to multiple threads (not a problem
> because there is proper locking, it's just a surprising aspect we should
> document).

OK.  Pointing out that this is just a relevant for performance and not a
correctness issue would be good.