Messages in this thread | | | Date | Fri, 10 Dec 2010 15:09:08 -0600 (CST) | From | Christoph Lameter <> | Subject | Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM |
| |
On Fri, 10 Dec 2010, Eric Dumazet wrote:
> > By the way, we need smp_wmb(), not barrier(), even only the "owner cpu" > can write into its 'percpu' seqcount. > > There is nothing special about a seqcount being percpu or a 'global' > one. We must have same memory barrier semantics.
There is certainly a major difference in that execution of a stream of instructions on the same cpu is guaranteed to have a coherent view of the data. That is not affected by interrupts etc.
> > this_cpu_write_seqcount_begin(&myseqcount); > this_cpu_add(mydata1, add1); > this_cpu_add(mydata2, add2); > this_cpu_inc(mydata3); > this_cpu_write_seqcount_end(&myseqcount); > > We protect the data[1,2,3] set with a seqcount, so need smp_wmb() in > both _begin() and _end()
There is nothing to protect there since processing is on the same cpu. The data coherency guarantees of the processor will not allow anything out of sequence to affect execution. An interrupt f.e. will not cause updates to mydata1 to get lost.
| |