Messages in this thread | | | Date | Wed, 17 Jun 2009 17:19:37 +0900 | From | Tejun Heo <> | Subject | Re: [this_cpu_xx 01/11] Introduce this_cpu_ptr() and generic this_cpu_* operations |
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Hello,
cl@linux-foundation.org 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?
Thanks.
-- tejun
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