Messages in this thread | | | Date | Wed, 29 Jan 2003 17:41:33 -0800 | From | Richard Henderson <> | Subject | Re: frlock and barrier discussion |
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On Wed, Jan 29, 2003 at 05:15:55PM -0800, Stephen Hemminger wrote: > First, write_begin/end can only be safely used when there is separate > writer synchronization such as a spin_lock or semaphore. > As far as I know, semaphore or spin_lock guarantees a barrier. > So xtime or anything else can not be read before the spin_lock. > > Using mb() is more paranoid than necessary.
If you want stuff to happen *between* the write_begin/end, or indeed for the begin/end not to be interleaved, then mb() is absolutely necessary. The most likely dynamic reordering of
//begin t1 = rw->pre_sequence t1 += 1 rw->pre_sequence = t1 wmb()
//stuff xtimensec = xtime.tv_nsec
//end wmb() t2 = rw->post_sequence t2 += 1 rw->post_sequence = t2
is
t1 = rw->pre_sequence t2 = rw->post_sequence xtimensec = xtime.tv_nsec t1 += 1; t2 += 2; rw->pre_sequence = t1 wmb() wmb() rw->post_sequence = t2
Why? Because pre_sequence and post_sequence are in the same cache line, and both reads could be satisfied in the same cycle by the same line fill from main memory.
If you don't care about stuff happening in between the write_begin/end, then why are you using them at all?
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