On Wed 2014-05-07 16:33:20, Jan Kara wrote: > On Wed 07-05-14 11:13:56, Petr Mládek wrote: > > On Mon 2014-05-05 19:18:46, Steven Rostedt wrote: > > > To prevent deadlocks with doing a printk inside the scheduler, > > > printk_sched() was created. The issue is that printk has a console_sem > > > that it can grab and release. The release does a wake up if there's a > > > task pending on the sem, and this wake up grabs the rq locks that is > > > held in the scheduler. This leads to a possible deadlock if the wake up > > > uses the same rq as the one with the rq lock held already. > > > > > > What printk_sched() does is to save the printk write in a per cpu buffer > > > and sets the PRINTK_PENDING_SCHED flag. On a timer tick, if this flag is > > > set, the printk() is done against the buffer. > > > > > > There's a couple of issues with this approach. > > > > > > 1) If two printk_sched()s are called before the tick, the second one > > > will overwrite the first one. > > > > > > 2) The temporary buffer is 512 bytes and is per cpu. This is a quite a > > > bit of space wasted for something that is seldom used. > > > > > > In order to remove this, the printk_sched() can use the printk buffer > > > instead, and delay the console_trylock()/console_unlock() to the queued > > > work. > > > > > > Because printk_sched() would then be taking the logbuf_lock, the > > > logbuf_lock must not be held while doing anything that may call into the > > > scheduler functions, which includes wake ups. Unfortunately, printk() > > > also has a console_sem that it uses, and on release, the > > > up(&console_sem) may do a wake up of any pending waiters. This must be > > > avoided while holding the logbuf_lock. > > > > > > Signed-off-by: Steven Rostedt <rost...@goodmis.org> > > > --- > > > This version has been forward ported to the 3.15-rc releases. > > > --- > ... > > > @@ -2440,18 +2470,20 @@ > > > #define PRINTK_BUF_SIZE 512 > > > > > > #define PRINTK_PENDING_WAKEUP 0x01 > > > -#define PRINTK_PENDING_SCHED 0x02 > > > +#define PRINTK_PENDING_OUTPUT 0x02 > > > > > > static DEFINE_PER_CPU(int, printk_pending); > > > -static DEFINE_PER_CPU(char [PRINTK_BUF_SIZE], printk_sched_buf); > > > > > > static void wake_up_klogd_work_func(struct irq_work *irq_work) > > > { > > > int pending = __this_cpu_xchg(printk_pending, 0); > > > > > > - if (pending & PRINTK_PENDING_SCHED) { > > > - char *buf = __get_cpu_var(printk_sched_buf); > > > - pr_warn("[sched_delayed] %s", buf); > > > + if (pending & PRINTK_PENDING_OUTPUT) { > > > + if (console_trylock()) > > > + console_unlock(); > > > > I wonder if we should call here console_trylock_for_printk() which checks > > whether the console is really usable. > So Stephen couldn't use console_trylock_for_printk() because that expects > logbuf_lock to be locked in vanilla kernel. Only after locking changes I > did it would be usable.
Ah yes, I meant to use console_trylock_for_printk() from current linux-next git tree. I am sorry, I should have been more explicit. > > The check for usable console was introduced in the commit > > 76a8ad293912cd2f (Make printk work for really early debugging). > > I think that this IRQ work could get called during early boot, > > so the check would make sense here as well. Or have I missed something? > I'm not really sure if IRQ work can be run on CPU which is not online. It would make sense. I was just curious because console_trylock_for_printk() was previously indirectly used via pr_warn(). Best Regards, Petr -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/