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

[Florian Weimer]

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).

Florian