On Tue, 6 May 2014 11:45:57 +0200 Jan Kara <j...@suse.cz> wrote: > On Mon 05-05-14 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> > Andrew already has this patch in -mm tree AFAIK...
Ah, and somebody told me that it wasn't in linux-next. But looking at it now, it seems to be. Thanks! -- Steve -- 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/