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SubjectRe: [PATCH v2] tracing: Choose static tp_printk buffer by explicit nesting count
Hi Andy,

On Fri, May 27, 2016 at 4:00 AM, Andy Lutomirski <luto@kernel.org> 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 <luto@kernel.org>

Maybe put_trace_buf() could take the buffer as an argument and ignore
if it's NULL. This way the error handling will be more simplified.
But your code is not very complicated so I won't insist strongly. So,

Acked-by: Namhyung Kim <namhyung@kernel.org>

Thanks,
Namhyung


>
> Changes from v1: Fix error handling (Namhyung Kim)
>
> 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..b3d703bda0a9 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;
> -static struct trace_buffer_struct *trace_percpu_sirq_buffer;
> -static struct trace_buffer_struct *trace_percpu_irq_buffer;
> -static struct trace_buffer_struct *trace_percpu_nmi_buffer;
>
> /*
> - * The buffer used is dependent on the context. There is a per cpu
> - * buffer for normal context, softirq contex, hard irq context and
> - * for NMI context. Thise allows for lockless recording.
> - *
> - * Note, if the buffers failed to be allocated, then this returns NULL
> + * 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 *percpu_buffer;
> -
> - /*
> - * If we have allocated per cpu buffers, then we do not
> - * need to do any locking.
> - */
> - if (in_nmi())
> - percpu_buffer = trace_percpu_nmi_buffer;
> - else if (in_irq())
> - percpu_buffer = trace_percpu_irq_buffer;
> - else if (in_softirq())
> - percpu_buffer = trace_percpu_sirq_buffer;
> - else
> - percpu_buffer = trace_percpu_buffer;
> + struct trace_buffer_struct *buffer = this_cpu_ptr(trace_percpu_buffer);
>
> - if (!percpu_buffer)
> + if (!buffer || buffer->nesting >= 4)
> return NULL;
>
> - return this_cpu_ptr(&percpu_buffer->buffer[0]);
> + return &buffer->buffer[buffer->nesting++][0];
> +}
> +
> +static void put_trace_buf(void)
> +{
> + this_cpu_dec(trace_percpu_buffer->nesting);
> }
>
> static int alloc_percpu_trace_buffer(void)
> {
> struct trace_buffer_struct *buffers;
> - struct trace_buffer_struct *sirq_buffers;
> - struct trace_buffer_struct *irq_buffers;
> - struct trace_buffer_struct *nmi_buffers;
>
> buffers = alloc_percpu(struct trace_buffer_struct);
> - if (!buffers)
> - goto err_warn;
> -
> - sirq_buffers = alloc_percpu(struct trace_buffer_struct);
> - if (!sirq_buffers)
> - goto err_sirq;
> -
> - irq_buffers = alloc_percpu(struct trace_buffer_struct);
> - if (!irq_buffers)
> - goto err_irq;
> -
> - nmi_buffers = alloc_percpu(struct trace_buffer_struct);
> - if (!nmi_buffers)
> - goto err_nmi;
> + if (WARN(!buffers, "Could not allocate percpu trace_printk buffer"))
> + return -ENOMEM;
>
> trace_percpu_buffer = buffers;
> - trace_percpu_sirq_buffer = sirq_buffers;
> - trace_percpu_irq_buffer = irq_buffers;
> - trace_percpu_nmi_buffer = nmi_buffers;
> -
> return 0;
> -
> - err_nmi:
> - free_percpu(irq_buffers);
> - err_irq:
> - free_percpu(sirq_buffers);
> - err_sirq:
> - free_percpu(buffers);
> - err_warn:
> - WARN(1, "Could not allocate percpu trace_printk buffer");
> - return -ENOMEM;
> }
>
> static int buffers_allocated;
> @@ -2153,7 +2111,7 @@ int trace_vbprintk(unsigned long ip, const char *fmt, va_list args)
> tbuffer = get_trace_buf();
> if (!tbuffer) {
> len = 0;
> - goto out;
> + goto out_nobuffer;
> }
>
> len = vbin_printf((u32 *)tbuffer, TRACE_BUF_SIZE/sizeof(int), fmt, args);
> @@ -2179,6 +2137,9 @@ int trace_vbprintk(unsigned long ip, const char *fmt, va_list args)
> }
>
> out:
> + put_trace_buf();
> +
> +out_nobuffer:
> preempt_enable_notrace();
> unpause_graph_tracing();
>
> @@ -2210,7 +2171,7 @@ __trace_array_vprintk(struct ring_buffer *buffer,
> tbuffer = get_trace_buf();
> if (!tbuffer) {
> len = 0;
> - goto out;
> + goto out_nobuffer;
> }
>
> len = vscnprintf(tbuffer, TRACE_BUF_SIZE, fmt, args);
> @@ -2229,7 +2190,11 @@ __trace_array_vprintk(struct ring_buffer *buffer,
> __buffer_unlock_commit(buffer, event);
> ftrace_trace_stack(&global_trace, buffer, flags, 6, pc, NULL);
> }
> - out:
> +
> +out:
> + put_trace_buf();
> +
> +out_nobuffer:
> preempt_enable_notrace();
> unpause_graph_tracing();
>
> --
> 2.5.5
>

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