Re: [PATCH v2] lock/semaphore: Avoid an unnecessary deadlock within up()

[Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>]

On (02/03/16 09:04), Ingo Molnar wrote:
> * Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> 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
>