On (02/03/16 09:04), Ingo Molnar wrote:
> * 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.
yes, correct. I just said for the info that there is already 'move printk away
from
console_sem' work in progress. Well, the reason for that work is entirely
different,
though, but this console_sem recursion and console driver's lock recursion can
be
'fixed as a side effect'.
> Furthermore, logbuf_lock already has recursion protection:
>
> /*
> * Ouch, printk recursed into itself!
> */
> if (unlikely(logbuf_cpu == this_cpu)) {
it's good, no doubt. but it doesn't work in all of the cases. a simple one is:
vprintk_emit()
...
raw_spin_lock(&logbuf_lock);
logbuf_cpu = this_cpu;
...
logbuf_cpu = UINT_MAX;
raw_spin_unlock(&logbuf_lock); << SPIN_BUG_ON
...
if raw_spin_unlock() calls SPIN_BUG_ON, then logbuf_lock recursion detection
can't
help. we recurse into vprintk_emit() with logbuf_lock locked and logbuf_cpu !=
this_cpu.
Peter Hurley also posted the following case (I'll quote):
serial8250_do_set_termios()
spin_lock_irqsave() ** claim port lock **
...
serial_port_out(port, UART_LCR, ....);
dw8250_serial_out()
dev_err()
vprintk_emit()
console_trylock()
call_console_drivers()
serial8250_console_write()
spin_lock_irqsave() ** port lock **
** DEADLOCK **
-ss
> 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
>
