Comment 17 for bug 425723

In multi-threaded programs, we are seeing a lot of free() aborts with
MALLOC_CHECK_ turned on (our default settings) with glibc-2.10 on
openSUSE:Factory. A simple testcase is not easy to make, but I suppose
brute-forcing parallel free()s agressively enough would make it show up.

I think this locking change is the cause. In realloc_check(), the mutex is
explicitly taken when calling mem2chunk_check(), and mem2chunk_check appears to
be accessing other parts of the arena which I guess is unsafe without the mutex.

Shouldn't the mutex be held during mem2chunk_check()?