Re: [this_cpu_xx 01/11] Introduce this_cpu_ptr() and generic this_cpu_*operations

From: Tejun Heo
Date: Wed Jun 17 2009 - 04:20:57 EST


cl@xxxxxxxxxxxxxxxxxxxx wrote:
> +#ifndef this_cpu_write
> +# define this_cpu_write(pcp, val) __this_cpu_write((pcp), (val))
> +#endif

Is this safe? Write itself would always be atomic but this means that
a percpu variable may change its value while a thread is holding the
processor by disabling preemption. ie,

0. v contains A for cpu0

1. task0 on cpu0 does this_cpu_write(v, B), looks up cpu but gets
preemted out.

2. task1 gets scheduled on cpu1, disables preemption and does
__this_cpu_read(v) and gets A and goes on with preemtion disabled.

3. task0 gets scheduled on cpu1 and executes the assignment.

4. task1 does __this_cpu_read(v) again and oops gets B this time.

Please note that this can also happen between addition or other
modifying ops and cause incorrect result.

Also, these macros depricate percpu_OP() macros, right?


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