Messages in this thread | | | Date | Mon, 27 Nov 2006 16:10:27 -0500 (EST) | From | Alan Stern <> | Subject | Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync |
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On Mon, 27 Nov 2006, Oleg Nesterov wrote:
> I still can't relax, another attempt to "prove" this should not be > possible on CPUs supported by Linux :) > > Let's suppose it is possible, then it should also be possible if CPU_1 > does spin_lock() instead of mb() (spin_lock can't be "stronger"), yes? > > Now, > > int COND; > wait_queue_head_t wq; > > my_wait() > { > add_wait_queue(&wq); > for (;;) { > set_current_state(TASK_UNINTERRUPTIBLE); > > if (COND) > break; > > schedule(); > } > remove_wait_queue(&wq); > } > > my_wake() > { > COND = 1; > wake_up(&wq); > } > > this should be correct, but it is not! > > my_wait: > > task->state = TASK_UNINTERRUPTIBLE; // STORE > > mb(); > > if (COND) break; // LOAD > > > my_wake: > > COND = 1; // STORE > > spin_lock(WQ.lock); > spin_lock(runqueue.lock); > > // try_to_wake_up() > if (!(task->state & TASK_UNINTERRUPTIBLE)) // LOAD > goto out; > > > So, my_wait() gets COND == 0, and goes to schedule in 'D' state. > try_to_wake_up() reads ->state == TASK_RUNNING, and does nothing.
This is a very good point. I don't know what the resolution is; Paul will have to explain the situation.
Alan
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