On Mon, 4 Sep 2017 14:22:46 +0900
Sergey Senozhatsky <sergey.senozhatsky.w...@gmail.com> wrote:
> like I said in another email, printk-safe buffer
> is per-CPU and is also used for actual printk-safe, hence it must be
> used with local IRQs disabled when we "borrow" the buffer for pr_line
> (disabled preemption is not enough due to possible IRQ printk-safe
> print out). this can be a bit annoying.
You can do what I did with trace_printk(). I have a buffer per context.
Then you only need to use preempt_disable() to do the print. That is,
trace_printk() has 4 buffers:
1. Normal context
2. softirq context
3. irq context
4. NMI context
It determines which context it is in, disables preemption, and uses the
corresponding buffer. This way I don't need to worry about being
preempted by an interrupt or NMI.
Grant it, it does make the memory needed 4x bigger.
I have an array of 4 buffers, and the following code:
static char *get_trace_buf(void)
{
struct trace_buffer_struct *buffer = this_cpu_ptr(trace_percpu_buffer);
if (!buffer || buffer->nesting >= 4)
return NULL;
return &buffer->buffer[buffer->nesting++][0];
}
Hmm, I probably need to add a "barrier()" before the return, or use a
this_cpu_inc() on nesting. As long as the nesting variable is updated
before the return of the buffer being used, then everything is fine.
Because we have:
static void put_trace_buf(void)
{
this_cpu_dec(trace_percpu_buffer->nesting);
}
And anything that preempts this call will have returned it back to its
original state before returning.
-- Steve
