11–20 of 28 posts
Not safe, do not pass go, do not collect $200, absolutely do not use.
[1] https://github.com/sirgallo/cmapv2/blob/280e3017ae4ba212f6f8...
This repo is completely unsound, code like [1] is pervasive and demonstrates a total misunderstanding of what guarantees are provided -- or, really, not provided -- by "atomic" reads of unsafe.Pointer values. Data races everywhere! Not safe, do not pass go, do not collect $200, absolutely do not use. [1] https://github.com/sirgallo/cmapv2/blob/280e3017ae4ba212f6f8...
---
CopyNode broken
`CopyNode` duplicates only the parent; every child pointer is still shared
nodeCopy.setChildren(make([]*node, len(n.children)))
copy(nodeCopy.children, n.children) // pointers reused
Any later mutation (for example `setValue`) writes through those shared pointers, so readers and historical snapshots are modified concurrently -- invalid, data race, memory model violation
---
Bitmap corruption
`SetBit` uses XOR rather than “set”:
func SetBit(bitmap uint32, position int) uint32 { return bitmap ^ (1 Calling it twice on the same index flips the bit back to 0. During branch-creation on insert and during delete, this function is invoked multiple times on the same index, clearing a bit that should remain set and leaving orphaned children.
---
Invalid assumptions re: interior pointers
Only the root pointer is read with `atomic.LoadPointer`. All deeper fields like `children[pos]`, `bitmap`, and the byte-slice keys/values, are accessed directly after a successful CAS. Readers therefore race with writers that mutate these fields in place -- race condition, memory model violation, etc.
pos := cMap.getPosition(node.Bitmap(), hash, level)
if node.Child(pos).IsLeaf() && bytes.Equal(key, node.Child(pos).Key()) {
return node.Child(pos).Value()
}
---
All xxxRecursive functions rely on those invalid interior pointer assumptions
Sequence in `putRecursive` / `deleteRecursive` is
1. `curr := atomic.LoadPointer(ptr)`
2. Build `nodeCopy`
3. Recurse; grandchildren are mutated in place
4. `atomic.CompareAndSwap(ptr, curr, nodeCopy)`
If another goroutine has already swapped in a different copy of `curr` (and mutated it) the CAS still succeeds because the pointer value is unchanged, merging incompatible sub-tries and corrupting the data
---
Use-after-free in sync.Pool
On CAS failure the freshly built `nodeCopy` is immediately returned to a `sync.Pool` -- undefined behavior
cMap.pool.PutNode(nodeCopy) // may race with outstanding readers
Other goroutines still holding references to that node can now access a reclaimed object, oops.
---
K/V Aliasing
Keys and values (both []byte slices, which are not safe for concurrent r/w access) are stored by reference, a mistake:
n.setKey(key)
n.setValue(value)
---
Reader panics, etc.
- `getRecursive` accesses `children[pos]` without bounds or nil checks, concurrent shrink can make `pos` invalid
- `GetIndex` allows a negative `shiftSize` once `level >= 7` with `chunkSize = 5`, producing nonsense indices and potential slice-out-of-boundsThis repo is completely unsound, code like [1] is pervasive and demonstrates a total misunderstanding of what guarantees are provided -- or, really, not provided -- by "atomic" reads of unsafe.Pointer values. Data races everywhere! Not safe, do not pass go, do not collect $200, absolutely do not use. [1] https://github.com/sirgallo/cmapv2/blob/280e3017ae4ba212f6f8...
This repo is completely unsound, code like [1] is pervasive and demonstrates a total misunderstanding of what guarantees are provided -- or, really, not provided -- by "atomic" reads of unsafe.Pointer values. Data races everywhere! Not safe, do not pass go, do not collect $200, absolutely do not use. [1] https://github.com/sirgallo/cmapv2/blob/280e3017ae4ba212f6f8...
Be more specific? OK --- CopyNode broken `CopyNode` duplicates only the parent; every child pointer is still shared nodeCopy.setChildren(make([]*node, len(n.children))) copy(nodeCopy.children, n.children) // pointers reused https://github.com/sirgallo/cmapv2/blob/main/node.go#L11-L17 Any later mutation (for example `setValue`) writes through those shared pointers, so readers and historical snapshots are modified conc…
You must clone the slice on both write and read, right?
I get that cloning incurs a memory allocation and a copy operation, but this is the price for safety when concurrent access is possible or your data may be bodified outside your structure.
You could probably intern immutable keys, or avoid storing if keys already exist and are immutable, or use an object pool (like sync.Pool) to reduce allocations if this happens at scale. Anything else I am missing?
This repo is completely unsound, code like [1] is pervasive and demonstrates a total misunderstanding of what guarantees are provided -- or, really, not provided -- by "atomic" reads of unsafe.Pointer values. Data races everywhere! Not safe, do not pass go, do not collect $200, absolutely do not use. [1] https://github.com/sirgallo/cmapv2/blob/280e3017ae4ba212f6f8...
Thank you for debunking it so I didn't have to. I don't think I've ever seen someone post a low-level/concurrent data structure in Go that wasn't wildly unsound, so I assumed this was too.
This repo is completely unsound, code like [1] is pervasive and demonstrates a total misunderstanding of what guarantees are provided -- or, really, not provided -- by "atomic" reads of unsafe.Pointer values. Data races everywhere! Not safe, do not pass go, do not collect $200, absolutely do not use. [1] https://github.com/sirgallo/cmapv2/blob/280e3017ae4ba212f6f8...
Thank you for debunking it so I didn't have to. I don't think I've ever seen someone post a low-level/concurrent data structure in Go that wasn't wildly unsound, so I assumed this was too.
Earlier quoted context omitted.
Thank you for debunking it so I didn't have to. I don't think I've ever seen someone post a low-level/concurrent data structure in Go that wasn't wildly unsound, so I assumed this was too.
Go is made by Google. Do they not have someone writing an equivalent of Java.util.concurrent.ConcurrentHashMap?
Earlier quoted context omitted.
Be more specific? OK --- CopyNode broken `CopyNode` duplicates only the parent; every child pointer is still shared nodeCopy.setChildren(make([]*node, len(n.children))) copy(nodeCopy.children, n.children) // pointers reused https://github.com/sirgallo/cmapv2/blob/main/node.go#L11-L17 Any later mutation (for example `setValue`) writes through those shared pointers, so readers and historical snapshots are modified conc…
> both []byte slices, which are not safe for concurrent r/w access You must clone the slice on both write and read, right? I get that cloning incurs a memory allocation and a copy operation, but this is the price for safety when concurrent access is possible or your data may be bodified outside your structure. You could probably intern immutable keys, or avoid storing if keys already exist and are immutable, or use a…
I haven't looked at the code, but that doesn't make sense to me. If you can't read the slice safely, you also can't clone it safely.
Earlier quoted context omitted.
> both []byte slices, which are not safe for concurrent r/w access You must clone the slice on both write and read, right? I get that cloning incurs a memory allocation and a copy operation, but this is the price for safety when concurrent access is possible or your data may be bodified outside your structure. You could probably intern immutable keys, or avoid storing if keys already exist and are immutable, or use a…
> You must clone the slice on both write and read, right? I haven't looked at the code, but that doesn't make sense to me. If you can't read the slice safely, you also can't clone it safely.
Like okay, I read "both []byte slices, which are not safe for concurrent r/w access", but then, what is the solution? If the claim is indeed true.