Date: Sat, 21 Apr 2018 01:52:36 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: What's wrong with musl's malloc On Sat, Apr 21, 2018 at 01:28:12AM -0400, Rich Felker wrote: > One protocol that might work: > > When locking multiple bins, they must be locked in decreasing order. > > For malloc, this is the natural thing to do -- the bin you return the > excess to will be the same or smaller than the bin you allocate from. > > For free, take the freelock from the beginning, rather than just > momentarily at the end. Then you know the bin sizes you'll be working ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > with (up to at most 3 -- self, prev, and next) and can lock them in ^^^^^^^^^^^^^^^^^^^^^ I don't think this is actually quite correct. A concurrent malloc could consume part of either of the adjacent free chunks. It's okay if it consumes the whole thing, but if it only consumes part, you'll end up with a new adjacent free chunk of a different size (different bin). Of course loop-and-retry is an option but doesn't seem like a good one; the retry time could be unbounded. Not sure if this is salvagable or not. Problems like this keep pointing in the direction of wanting to lock chunks (via their header/footer) rather than locking just bins. On the plus side it's much finer-grained locking and MT-friendly. Unfortunately it's also more complex and might have more fixed overhead. Rich
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