Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation

Sergey Senozhatsky
2019-06-26T02:08:44Z

On (06/25/19 14:03), John Ogness wrote:
[..]
> > CPU0 CPU1
> > printk(...)
> > sz = vscprintf(NULL, "Comm %s\n", current->comm);
> > ia64_mca_modify_comm()
> > snprintf(comm, sizeof(comm), "%s %d", current->comm, previous_current->pid);
> > memcpy(current->comm, comm, sizeof(current->comm));
> > if ((buf = prb_reserve(... sz))) {
> > vscnprintf(buf, "Comm %s\n", current->comm);
> > ^^^^^^^^^^^^^^ ->comm has changed.
> > Nothing critical, we
> > should not corrupt
> > anything, but we will
> > truncate ->comm if its
> > new size is larger than
> > what it used to be when
> > we did vscprintf(NULL).
> > prb_commit(...);
> > }

[..]
> In my v1 rfc series, I avoided this issue by having a separate dedicated
> ringbuffer (rb_sprintf) that was used to allocate a temporary max-size
> (2KB) buffer for sprinting to. Then _that_ was used for the real
> ringbuffer input (strlen, prb_reserve, memcpy, prb_commit). That would
> still be the approach of my choice.

In other words per-CPU buffering, AKA printk_safe ;)

-ss