On 19/03/15 18:30, Peter Zijlstra wrote:
On Thu, Mar 19, 2015 at 01:39:58PM -0400, Steven Rostedt wrote:
+void printk_nmi_backtrace_complete(void) +{
- struct nmi_seq_buf *s;
- int len, cpu, i, last_i;
- /*
* Now that all the NMIs have triggered, we can dump out their* back traces safely to the console.*/- for_each_possible_cpu(cpu) {
s = &per_cpu(nmi_print_seq, cpu);last_i = 0;len = seq_buf_used(&s->seq);if (!len)continue;/* Print line by line. */for (i = 0; i < len; i++) {if (s->buffer[i] == '\n') {print_seq_line(s, last_i, i);last_i = i + 1;}}/* Check if there was a partial line. */if (last_i < len) {print_seq_line(s, last_i, len - 1);pr_cont("\n");}/* Wipe out the buffer ready for the next time around. */seq_buf_clear(&s->seq);- }
- clear_bit(0, &nmi_print_flag);
- smp_mb__after_atomic();
Is this really necessary. What is the mb synchronizing?
[ Added Peter Zijlstra to confirm it's not needed ]
It surely looks suspect; and it lacks a comment, which is a clear sign its buggy.
Now it if tries to order the accesses to the seqbuf againt the clearing of the bit one would have expected a _before_ barrier, not an _after_.
It's nothing to do with the seqbuf since I added the seqbuf code myself but the barrier was already in the code that I copied from.
In the mainline code today it looks like this as part of the x86 code (note that call to put_cpu() in my patchset but it lives in the arch/ specific code rather than the generic code):
: /* Check if there was a partial line. */ : if (last_i < len) { : print_seq_line(s, last_i, len - 1); : pr_cont("\n"); : } : } : : clear_bit(0, &backtrace_flag); : smp_mb__after_atomic(); : put_cpu(); : }
The barrier was not intended to have anything to do with put_cpu() either though since the barrier was added before put_cpu() arrived: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=5...
There's nothing in the commit comment explaining the barrier and I really can't see what it is for.
Daniel.