sync::rwlock has an obscure 3-participant race around access_lock that can result in arbitrary type coercion #7065
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A-concurrency
Area: Concurrency
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ghost
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bblum
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Jun 13, 2013
This was referenced Jun 13, 2013
bors
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Jun 15, 2013
r? @brson links to issues: #7065 the race that's fixed; #7066 the perf improvement I added. There are also some minor cleanup commits here. To measure the performance improvement from replacing the exclusive with an atomic uint, I edited the ```msgsend-ring-rw-arcs``` bench test to do a ```write_downgrade``` instead of just a ```write```, so that it stressed the code paths that accessed ```read_count```. (At first I was still using ```write``` and saw no performance difference whatsoever, whoooops.) The bench test measures how long it takes to send 1,000,000 messages by using rwarcs to emulate pipes. I also measured the performance difference imposed by the fix to the ```access_lock``` race (which involves taking an extra semaphore in the ```cond.wait()``` path). The net result is that fixing the race imposes a 4% to 5% slowdown, but doing the atomic uint optimization gives a 6% to 8% speedup. Note that this speedup will be most visible in read- or downgrade-heavy workloads. If an RWARC's only users are writers, the optimization doesn't matter. All the same, I think this more than justifies the extra complexity I mentioned in #7066. The raw numbers are: ``` with xadd read count before write_cond fix 4.18 to 4.26 us/message with write_cond fix 4.35 to 4.39 us/message with exclusive read count before write_cond fix 4.41 to 4.47 us/message with write_cond fix 4.65 to 4.76 us/message ```
flip1995
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Apr 22, 2021
…ishearth Add a note on the issue rust-lang#5953 Hello, I thought it would be better to have a note and warning about this issue considering it introduced an UB in the past even with the "Search on Github" feature. --- changelog: Add a note on the issue rust-lang#5953 to the known problems section.
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Figuring it out is left as an exercise to the reader.
(Of course, I will deal with it.)
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