Messages in this thread | | | Date | Thu, 13 Nov 2008 18:46:12 +0100 | From | "Frédéric Weisbecker" <> | Subject | Re: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless |
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2008/11/13 Steven Rostedt <rostedt@goodmis.org>: > > On Thu, 13 Nov 2008, Andi Kleen wrote: > >> > So the answer to this is: >> > >> > i = index++; >> > barrier(); >> > write to index i (not index); >> >> That was my first thought when I wrote the original email, >> but the disadvantage is that barrier() is a big hammer >> that flushes everything and can make the code much worse. >> That is why I suggested local_add_return() instead. > > barrier() is a compiler barrier, does nothing with the caches, and is > quite cheap. We only need a compiler barrier because we are only > protecting ourselves from things that happen on the current CPU. No other > devices or other CPUs are involved.
Oh I see the issue now. The value of the index could have been incremented in a register and not yet in the memory... So yes, a barrier() to make these operations flushed in memory before using the index.
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