Hi, assuming that this passes warn stall torturing by Tetsuo, do you think we can drop http://lkml.kernel.org/r/1509017339-4802-1-git-send-email-penguin-kernel@xxxxxxxxxxxxxxxxxxx from the mmotm tree? On Wed 08-11-17 10:27:23, Steven Rostedt wrote: > [ claws-mail is really pissing me off. It did it again, after I > manually fixed all the addresses. This time, I'm going to do things > slightly different. Sorry for all the spam :-( ] > > From: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx> > > This patch implements what I discussed in Kernel Summit. I added > lockdep annotation (hopefully correctly), and it hasn't had any splats > (since I fixed some bugs in the first iterations). It did catch > problems when I had the owner covering too much. But now that the owner > is only set when actively calling the consoles, lockdep has stayed > quiet. > > Here's the design again: > > I added a "console_owner" which is set to a task that is actively > writing to the consoles. It is *not* the same an the owner of the > console_lock. It is only set when doing the calls to the console > functions. It is protected by a console_owner_lock which is a raw spin > lock. > > There is a console_waiter. This is set when there is an active console > owner that is not current, and waiter is not set. This too is protected > by console_owner_lock. > > In printk() when it tries to write to the consoles, we have: > > if (console_trylock()) > console_unlock(); > > Now I added an else, which will check if there is an active owner, and > no current waiter. If that is the case, then console_waiter is set, and > the task goes into a spin until it is no longer set. > > When the active console owner finishes writing the current message to > the consoles, it grabs the console_owner_lock and sees if there is a > waiter, and clears console_owner. > > If there is a waiter, then it breaks out of the loop, clears the waiter > flag (because that will release the waiter from its spin), and exits. > Note, it does *not* release the console semaphore. Because it is a > semaphore, there is no owner. Another task may release it. This means > that the waiter is guaranteed to be the new console owner! Which it > becomes. > > Then the waiter calls console_unlock() and continues to write to the > consoles. > > If another task comes along and does a printk() it too can become the > new waiter, and we wash rinse and repeat! > > Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx> > --- > Changes from v3: > > Fixed while loop on console_waiter (Thanks Vlastimil!) > > Moved console_owner out of logbuf_lock taking (reported by > Tetsuo Handa) > > Changes from v2: > > - Added back some READ/WRITE_ONCE() just to be on the safe side > Index: linux-trace.git/kernel/printk/printk.c > =================================================================== > --- linux-trace.git.orig/kernel/printk/printk.c > +++ linux-trace.git/kernel/printk/printk.c > @@ -86,8 +86,15 @@ EXPORT_SYMBOL_GPL(console_drivers); > static struct lockdep_map console_lock_dep_map = { > .name = "console_lock" > }; > +static struct lockdep_map console_owner_dep_map = { > + .name = "console_owner" > +}; > #endif > > +static DEFINE_RAW_SPINLOCK(console_owner_lock); > +static struct task_struct *console_owner; > +static bool console_waiter; > + > enum devkmsg_log_bits { > __DEVKMSG_LOG_BIT_ON = 0, > __DEVKMSG_LOG_BIT_OFF, > @@ -1753,8 +1760,56 @@ asmlinkage int vprintk_emit(int facility > * semaphore. The release will print out buffers and wake up > * /dev/kmsg and syslog() users. > */ > - if (console_trylock()) > + if (console_trylock()) { > console_unlock(); > + } else { > + struct task_struct *owner = NULL; > + bool waiter; > + bool spin = false; > + > + printk_safe_enter_irqsave(flags); > + > + raw_spin_lock(&console_owner_lock); > + owner = READ_ONCE(console_owner); > + waiter = READ_ONCE(console_waiter); > + if (!waiter && owner && owner != current) { > + WRITE_ONCE(console_waiter, true); > + spin = true; > + } > + raw_spin_unlock(&console_owner_lock); > + > + /* > + * If there is an active printk() writing to the > + * consoles, instead of having it write our data too, > + * see if we can offload that load from the active > + * printer, and do some printing ourselves. > + * Go into a spin only if there isn't already a waiter > + * spinning, and there is an active printer, and > + * that active printer isn't us (recursive printk?). > + */ > + if (spin) { > + /* We spin waiting for the owner to release us */ > + spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_); > + /* Owner will clear console_waiter on hand off */ > + while (READ_ONCE(console_waiter)) > + cpu_relax(); > + > + spin_release(&console_owner_dep_map, 1, _THIS_IP_); > + printk_safe_exit_irqrestore(flags); > + > + /* > + * The owner passed the console lock to us. > + * Since we did not spin on console lock, annotate > + * this as a trylock. Otherwise lockdep will > + * complain. > + */ > + mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_); > + console_unlock(); > + printk_safe_enter_irqsave(flags); > + } > + printk_safe_exit_irqrestore(flags); > + > + } > } > > return printed_len; > @@ -2141,6 +2196,7 @@ void console_unlock(void) > static u64 seen_seq; > unsigned long flags; > bool wake_klogd = false; > + bool waiter = false; > bool do_cond_resched, retry; > > if (console_suspended) { > @@ -2229,14 +2285,64 @@ skip: > console_seq++; > raw_spin_unlock(&logbuf_lock); > > + /* > + * While actively printing out messages, if another printk() > + * were to occur on another CPU, it may wait for this one to > + * finish. This task can not be preempted if there is a > + * waiter waiting to take over. > + */ > + raw_spin_lock(&console_owner_lock); > + console_owner = current; > + raw_spin_unlock(&console_owner_lock); > + > + /* The waiter may spin on us after setting console_owner */ > + spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_); > + > stop_critical_timings(); /* don't trace print latency */ > call_console_drivers(ext_text, ext_len, text, len); > start_critical_timings(); > + > + raw_spin_lock(&console_owner_lock); > + waiter = READ_ONCE(console_waiter); > + console_owner = NULL; > + raw_spin_unlock(&console_owner_lock); > + > + /* > + * If there is a waiter waiting for us, then pass the > + * rest of the work load over to that waiter. > + */ > + if (waiter) > + break; > + > + /* There was no waiter, and nothing will spin on us here */ > + spin_release(&console_owner_dep_map, 1, _THIS_IP_); > + > printk_safe_exit_irqrestore(flags); > > if (do_cond_resched) > cond_resched(); > } > + > + /* > + * If there is an active waiter waiting on the console_lock. > + * Pass off the printing to the waiter, and the waiter > + * will continue printing on its CPU, and when all writing > + * has finished, the last printer will wake up klogd. > + */ > + if (waiter) { > + WRITE_ONCE(console_waiter, false); > + /* The waiter is now free to continue */ > + spin_release(&console_owner_dep_map, 1, _THIS_IP_); > + /* > + * Hand off console_lock to waiter. The waiter will perform > + * the up(). After this, the waiter is the console_lock owner. > + */ > + mutex_release(&console_lock_dep_map, 1, _THIS_IP_); > + printk_safe_exit_irqrestore(flags); > + /* Note, if waiter is set, logbuf_lock is not held */ > + return; > + } > + > console_locked = 0; > > /* Release the exclusive_console once it is used */ -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
