Fixes for ConcurrentDictionary for weak memory model hardware

[m08pvv-zz]

There was possibility to observe partially constructed object (Node) in ConcurrentDictionary on architectures with weak hardware memory model (e.g. ARM).
Volatile.Write protects us from exposing reference to partially constructed Node object.

[kouvel]

This looks good to me, but it seems like there may be more places where this would be an issue:

• GrowTable:

                        newBuckets[newBucketNo] = new Node(current._key, current._value, current._hashcode, newBuckets[newBucketNo]);

  After the array ref is updated with the new array, isn't there still a possibility that TryGetValue would see the new table and node but old values for node fields?
• Reading from and writing to chain links, for example in TryAddInternal but elsewhere as well:

                                    Node newNode = new Node(node._key, value, hashcode, node._next);
                                    if (prev == null)
                                    {
                                        ...
                                    }
                                    else
                                    {
                                        prev._next = newNode;
                                    }

  It seems possible that TryAdd updates a node with a new node without touching the head node, then TryGetValue sees the new node but old values for its fields.

What do you think?
CC @stephentoub

[m08pvv-zz]

@kouvel GrowTable is safe because newBuckets array is exposed with help of _tables = new Tables(newBuckets, newLocks, newCountPerLock); and _tables is declared as volatile, so _tables will be updated after any other preceding memory operations.

_next is also defined as volatile, so reference will be updated safely as well.

[kouvel]

Missed the volatile fields, thanks!

[kouvel]

@dotnet-bot test Linux x64 Release Build

Looks like a random failure on CentOS:

Unhandled Exception of Type Xunit.Sdk.InRangeException
Message :
Assert.InRange() Failure\r\nRange:  (00:00:00.0000001 - 00:00:00.1412111)\r\nActual: 00:00:00.2264035
Stack Trace :
   at System.Diagnostics.Tests.ActivityTests.DiagnosticSourceStartStop() in /mnt/j/workspace/dotnet_corefx/master/linux-TGroup_netcoreapp+CGroup_Release+AGroup_x64+TestOuter_false_prtest/src/System.Diagnostics.DiagnosticSource/tests/ActivityTests.cs:line 432