Vincent Snijders schrieb:
Michael Schnell wrote:
In another topic (now closed) Andrew described that a code similar to
HansPeter's example did run correctly on a dual core machine, but produced
errors on a machine with more cores.
I've not been reading every message. Definitive URL?
I cannot provide an URL (from old Thunderbird), so let me cite the
essential part of Andrew's message:
>>
Simply protecting a loop with a futex is not sufficient. Assigning
variables in a multi-threaded environment is not sufficient.
Interlocked/Exchange/ExchangeAdd/Increment features must be used
whenever dealing with variables (with at least one exception) In my
experience arrays of boolean are atomic.
I ran tests on a HexCore AMD system which had unexplained anomalies
with pointers in an bidirectional linked list. I replaced all the
pointer assignments with InterlockedExchange and the system worked
flawlessly. Some of the assignments were taking place in a
criticalsection (btw).
<<
This scenario was reflected in my example (bi-linked list update).
The very last sentence deserves clarification. When not *all*
assignments are protected by a CS, the use of Interlocked assignments
only can improve cache coherence, but it IMO *can not* prevent (logical)
race conditions. IMO Andrew observed just such race conditions,
resulting from a mix of sequential writes from multiple threads.
IMO a linked list can not work flawlessly, until at least a
multiple-read-exclusive-write lock is used. While an exclusive-write
lock can ensure list consistency, every unsynchronized reader will be
fooled by concurrent changes to the list structure :-(
Consider what will happen when an (unsynchronized) reader determined an
list node, before or after which a new node shall be inserted. When the
write-synchronized update will happen, the list structure already can
have changed, so that the referenced node can have changed its position
in the list, or can have been moved into a very different list :-(
DoDi
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