[Patch (preview)] libstdc++/24469

[Paolo Carlini]

Paolo Carlini wrote:

> I think everything is fine... Otherwise, please stop me before it's 
> too late! :)

Well, I can see a serious problem myself but it's fixable ;)

In a pure producer/consumer scheme, for example, some threads only 
allocate and some, different, threads only deallocate, thus _M_used of 
the allocating threads can easily overflow, especially so the atomic 
counters, which are only 32-bits wide. The solution, simple, is 
resyncing _M_used of each thread to its reclaimed counterpart at the 
beginning of both _M_reclaim_block and _M_reserve_block (the fast 
allocations, using _M_adjust_freelist are not affected and the binary 
compatibility still safe). In order not to affect the performance we can 
do that every fixed number of allocations and deallocations, say 1 << 12 
or something, and easily amortize the atomic... Will do tomorrow....

Paolo.