On Tue, May 24, 2016 at 03:52:28PM -0700, Andy Lutomirski wrote:
> Currently, the trace_printk code chooses which static buffer to use based
> on what type of atomic context (NMI, IRQ, etc) it's in. Simplify the
> code and make it more robust: simply count the nesting depth and choose
> a buffer based on the current nesting depth.
>
> The new code will only drop an event if we nest more than 4 deep,
> and the old code was guaranteed to malfunction if that happened.
>
> Signed-off-by: Andy Lutomirski <[email protected]>
> ---
> kernel/trace/trace.c | 83
> +++++++++++++++-------------------------------------
> 1 file changed, 24 insertions(+), 59 deletions(-)
>
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index a2f0b9f33e9b..4508f3bf4a97 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -1986,83 +1986,41 @@ static void __trace_userstack(struct trace_array *tr,
> unsigned long flags)
>
> /* created for use with alloc_percpu */
> struct trace_buffer_struct {
> - char buffer[TRACE_BUF_SIZE];
> + int nesting;
> + char buffer[4][TRACE_BUF_SIZE];
> };
>
> static struct trace_buffer_struct *trace_percpu_buffer;
> /*
> + * Thise allows for lockless recording. If we're nested too deeply, then
> + * this returns NULL.
> */
> static char *get_trace_buf(void)
> {
> + struct trace_buffer_struct *buffer = this_cpu_ptr(trace_percpu_buffer);
>
> + if (!buffer || buffer->nesting >= 4)
> return NULL;
This is buggy fwiw; you need to unconditionally increment
buffer->nesting to match the unconditional decrement.
Otherwise 5 'increments' and 5 decrements will land you at -1.
>
> + return &buffer->buffer[buffer->nesting++][0];
> +}
> +
> +static void put_trace_buf(void)
> +{
> + this_cpu_dec(trace_percpu_buffer->nesting);
> }
So I don't know about tracing; but for perf this construct would not
work 'properly'.
The per context counter -- which is lost in this scheme -- guards
against in-context recursion.
Only if we nest from another context do we allow generation of a new
event.
