Messages in this thread |  | | | Date | Fri, 1 May 2009 12:31:58 -0400 (EDT) | | From | Steven Rostedt <> | | Subject | Re: [PATCH 3/3] ring-buffer: make cpu buffer entries counter atomic |
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On Fri, 1 May 2009, Ingo Molnar wrote: > > > > > the counter too. This would cause missing entries to be added. > > > > > > > > > - unsigned long entries; > > > > > + atomic_t entries; > > > > > > > > Hm, that's not really good as atomics can be rather expensive and > > > > this is the fastpath. > > > > > > Actually, it could be local_t. I used that in a lot of the other places. > > > The race is with on CPU not other CPUs, and on archs like x86 there > > > is not cost of the "LOCK". > > > > Ug, it must be atomic_t. It is also modified by the reader. Thus > > it is not only a race with a single CPU but also multiple CPUs. > > > > This means that interrupts disabled is not the only proctection it > > needs. It must either be an atomic, or protected by a spinlock. > > Trace buffers are rather fundamentally per cpu. Where's the problem?
The entries keeps track of the number of entries in the buffer. A writer (producer) adds to the counter and readers (consumers) subtract from them. A writer can subtract them if it overwrites a page before the producer consumes it.
Only the writers are pinned to a CPU, the readers happen on any CPU.
-- Steve
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