[compiler-rt] [scudo] Allow to resize allocation ring buffer (PR #82683)

[Florian Mayer via llvm-commits]

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@@ -1594,12 +1600,33 @@ class Allocator {
     RB->StackDepotSize = StackDepotSize;
     RB->RawStackDepotMap = DepotMap;
 
-    atomic_store(&RingBufferAddress, reinterpret_cast<uptr>(RB),
-                 memory_order_release);
+    swapOutRingBuffer(RB);
     static_assert(sizeof(AllocationRingBuffer) %
                           alignof(typename AllocationRingBuffer::Entry) ==
                       0,
                   "invalid alignment");
+    return true;
+  }
+
+  void swapOutRingBuffer(AllocationRingBuffer *NewRB) {
+    // To allow resizeRingBuffer to be called in a multi-threaded context by apps,
+    // we do not actually unmap, but only madvise(DONTNEED) the pages. That way,
+    // straggler threads will not crash.
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fmayer wrote:

> And yes, the buffer swap is complicated under current design (Thanks for the email BTW). One easy way in my mind is, we only reset the available size of ring buffer and the ring buffer will be initialized with some value is big enough for all of their cases. As a result, in most cases, they still use small memory for the ring buffer.

That is not possible with the current design of the StackDepot as a hash table.

https://github.com/llvm/llvm-project/pull/82683