Messages in this thread | | | Date | Wed, 1 Dec 1999 16:27:25 +0100 | From | Jan Echternach <> | Subject | Re: Shared memory not SMP safe to user-mode code. |
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On Tue, Nov 30, 1999 at 10:50:56AM -0500, Richard B. Johnson wrote: > int main() > { > PARS *pars = NULL;
Shouldn't this be
volatile PARS *pars = NULL;
> volatile unsigned int key;
I don't think volatile is necessary here. key isn't shared.
> pars->spin += key; > if(pars->spin != key) > { > pars->spin -= key; /* didn't get spinlock */ > usleep(rand() % 10000); > continue; > } /* got spinlock; critical region */ > pars->spin -= key;
Let's assume process 1 has key == 50 and process 2 has key == 60. The `+=' operator shall be implemented as load register, add to register, store register.
P1 P2 spin -------------------- -------------------- ---------- 0 load spin -> 0 load spin -> 0 add key -> 60 store spin 60 add key -> 50 store spin 50 load spin -> 50 compare -> didn't get lock load spin -> 50 subtract key -> -10 store spin -10 load spin -> -10 compare -> didn't get lock load spin -> -10 subtract key -> -60 store spin -60
Do you see the problem? The algorithm requires atomic add/sub operations. Otherwise undoing modifications to pars->spin is not possible. Atomic load/store is not enough.
-- Jan
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