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tracing: Add barrier to trace_printk() buffer nesting modification
commit 3d9622c upstream. trace_printk() uses 4 buffers, one for each context (normal, softirq, irq and NMI), such that it does not need to worry about one context preempting the other. There's a nesting counter that gets incremented to figure out which buffer to use. If the context gets preempted by another context which calls trace_printk() it will increment the counter and use the next buffer, and restore the counter when it is finished. The problem is that gcc may optimize the modification of the buffer nesting counter and it may not be incremented in memory before the buffer is used. If this happens, and the context gets interrupted by another context, it could pick the same buffer and corrupt the one that is being used. Compiler barriers need to be added after the nesting variable is incremented and before it is decremented to prevent usage of the context buffers by more than one context at the same time. Cc: Andy Lutomirski <[email protected]> Fixes: e2ace00 ("tracing: Choose static tp_printk buffer by explicit nesting count") Hat-tip-to: Peter Zijlstra <[email protected]> Signed-off-by: Steven Rostedt (VMware) <[email protected]> Signed-off-by: Greg Kroah-Hartman <[email protected]>
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