* Sergey Senozhatsky <[email protected]> wrote:
> On (02/03/16 08:28), Ingo Molnar wrote:
> [..]
> > So why not move printk away from semaphores? Semaphores are classical
> > constructs
> > that have legacies and are somewhat non-obvious to use, compared to modern,
> > simpler locking primitives. I'd not touch their implementation, unless we
> > are
> > absolutely sure this is a safe optimization.
>
> semaphore's spin_lock is not the only spin lock that printk acquires. it also
> takes the logbuf_lock (and different locks in console drivers (up to console
> driver)).
>
> Jan Kara posted a patch that offloads printing job
> (console_trylock()-console_unlock()) from printk() call (when printk can
> offload
> it). so semaphore and console driver's locks will go away (mostly) with Jan's
> patch. logbug spin_lock, however, will stay.
Well, but this patch of yours only affects the semaphore code, so it does not
change the logbuf_lock situation.
Furthermore, logbuf_lock already has recursion protection:
/*
* Ouch, printk recursed into itself!
*/
if (unlikely(logbuf_cpu == this_cpu)) {
so it should not be possible to re-enter the printk() logbuf_lock critical
section
from the spinlock code. (There are other ways to get the logbuf_lock - if those
are still triggerable then they should be fixed.)
In any case, recursion protection is generally done in the debugging facilities
trying to behave lockless.
Thanks,
Ingo
