Messages in this thread | | | Subject | Re: [PATCH] small fixes in brlock.h | From | Robert Love <> | Date | 09 Mar 2003 19:00:01 -0500 |
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On Sun, 2003-03-09 at 18:44, Zwane Mwaikambo wrote:
> --- linux-2.5.64-unwashed/include/linux/brlock.h 5 Mar 2003 05:07:54 -0000 1.1.1.1 > +++ linux-2.5.64-unwashed/include/linux/brlock.h 9 Mar 2003 23:42:26 -0000 > @@ -85,8 +85,7 @@ > if (idx >= __BR_END) > __br_lock_usage_bug(); > > - preempt_disable(); > - _raw_read_lock(&__brlock_array[smp_processor_id()][idx]); > + read_lock(&__brlock_array[smp_processor_id()][idx]); > }
This is wrong.
We have to disable preemption prior to reading smp_processor_id(). Otherwise we may lock/unlock the wrong processor's brlock. The above as you changed it is equivalent to:
cpu = smp_processor_id(); /* do not want to preempt here, but we can! */ preempt_disable(); _raw_read_lock(&__brlock_array[cpu][idx]);
And what we had, and what we need, is:
preempt_disable(); cpu = smp_processor_id(); _raw_read_lock(&__brlock_array[cpu][idx]);
In other words, we need to ensure preemption is disabled prior to calling smp_processor_id().
We also do something similar with the write patch in lib/brlock.c, but for different reason: to disable preemption once and not NR_CPUS times.
The rest of the patch looks fine. :)
Robert Love
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