Messages in this thread | | | Date | Wed, 16 Jan 2002 22:33:58 +0100 | From | "R. Sinoradzki" <> | Subject | Re: multithreading on a multiprocessor system ( a bit OT ) |
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Justin Carlson wrote:
> On Wed, 2002-01-16 at 14:49, R. Sinoradzki wrote: > >>O.K my question: >>Consider two modern processors that share some data and a lock. >>The lock may be implemented with something like an atomic test-and-set >>instruction. Now processor 'A' acquires the lock and works with the data. >>Processor 'B' also wants to access the data, but internally reorders it's >>instructions because the instructions seem independent from each other. >>So 'B' might access the data without having the lock. >>If it's a single processor system, reordering instructions in a way that >>ensures that it looks 'as if' everything has been executed in the right order >>might be easy, but in a multiprocessor system 'A' doesn't know 'B's state. >> > > Then you've got a bug. Modern implementations that do SMP provide some > way of placing barriers around speculative execution structures to make > sure you don't, say, go out and read some memory location that changes > state in a device because that's an OK speculative action to take. > > Can't really comment on x86, as I'm not very good with it, but taking > for example MIPS and Alpha, in addition to the ll-sc ops, there are a > sync and mb instructions, respectively, which provide a method for > assuring that previous operations have become visible in terms of > general machine state before going on. > > -Justin >
Ah, thank you for the keywords. Sorry, I should have searched "multiprocessor synchronization" in Google, but I tried something else that gave me a lot of useless results ...
Ralf
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