Messages in this thread | | | Date | Wed, 27 Apr 2022 09:38:06 +0200 | From | Petr Mladek <> | Subject | Re: [PATCH printk v5 1/1] printk: extend console_lock for per-console locking |
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On Wed 2022-04-27 09:08:33, Marek Szyprowski wrote: > Hi, > > On 26.04.2022 15:16, Petr Mladek wrote: > > On Tue 2022-04-26 14:07:42, Petr Mladek wrote: > >> On Mon 2022-04-25 23:04:28, John Ogness wrote: > >>> Currently threaded console printers synchronize against each > >>> other using console_lock(). However, different console drivers > >>> are unrelated and do not require any synchronization between > >>> each other. Removing the synchronization between the threaded > >>> console printers will allow each console to print at its own > >>> speed. > >>> > >>> But the threaded consoles printers do still need to synchronize > >>> against console_lock() callers. Introduce a per-console mutex > >>> and a new console boolean field @blocked to provide this > >>> synchronization. > >>> > >>> console_lock() is modified so that it must acquire the mutex > >>> of each console in order to set the @blocked field. Console > >>> printing threads will acquire their mutex while printing a > >>> record. If @blocked was set, the thread will go back to sleep > >>> instead of printing. > >>> > >>> The reason for the @blocked boolean field is so that > >>> console_lock() callers do not need to acquire multiple console > >>> mutexes simultaneously, which would introduce unnecessary > >>> complexity due to nested mutex locking. Also, a new field > >>> was chosen instead of adding a new @flags value so that the > >>> blocked status could be checked without concern of reading > >>> inconsistent values due to @flags updates from other contexts. > >>> > >>> Threaded console printers also need to synchronize against > >>> console_trylock() callers. Since console_trylock() may be > >>> called from any context, the per-console mutex cannot be used > >>> for this synchronization. (mutex_trylock() cannot be called > >>> from atomic contexts.) Introduce a global atomic counter to > >>> identify if any threaded printers are active. The threaded > >>> printers will also check the atomic counter to identify if the > >>> console has been locked by another task via console_trylock(). > >>> > >>> Note that @console_sem is still used to provide synchronization > >>> between console_lock() and console_trylock() callers. > >>> > >>> A locking overview for console_lock(), console_trylock(), and the > >>> threaded printers is as follows (pseudo code): > >>> > >>> console_lock() > >>> { > >>> down(&console_sem); > >>> for_each_console(con) { > >>> mutex_lock(&con->lock); > >>> con->blocked = true; > >>> mutex_unlock(&con->lock); > >>> } > >>> /* console_lock acquired */ > >>> } > >>> > >>> console_trylock() > >>> { > >>> if (down_trylock(&console_sem) == 0) { > >>> if (atomic_cmpxchg(&console_kthreads_active, 0, -1) == 0) { > >>> /* console_lock acquired */ > >>> } > >>> } > >>> } > >>> > >>> threaded_printer() > >>> { > >>> mutex_lock(&con->lock); > >>> if (!con->blocked) { > >>> /* console_lock() callers blocked */ > >>> > >>> if (atomic_inc_unless_negative(&console_kthreads_active)) { > >>> /* console_trylock() callers blocked */ > >>> > >>> con->write(); > >>> > >>> atomic_dec(&console_lock_count); > >>> } > >>> } > >>> mutex_unlock(&con->lock); > >>> } > >>> > >>> The console owner and waiter logic now only applies between contexts > >>> that have taken the console_lock via console_trylock(). Threaded > >>> printers never take the console_lock, so they do not have a > >>> console_lock to handover. Tasks that have used console_lock() will > >>> block the threaded printers using a mutex and if the console_lock > >>> is handed over to an atomic context, it would be unable to unblock > >>> the threaded printers. However, the console_trylock() case is > >>> really the only scenario that is interesting for handovers anyway. > >>> > >>> @panic_console_dropped must change to atomic_t since it is no longer > >>> protected exclusively by the console_lock. > >>> > >>> Since threaded printers remain asleep if they see that the console > >>> is locked, they now must be explicitly woken in __console_unlock(). > >>> This means wake_up_klogd() calls following a console_unlock() are > >>> no longer necessary and are removed. > >>> > >>> Also note that threaded printers no longer need to check > >>> @console_suspended. The check for the @blocked field implicitly > >>> covers the suspended console case. > >>> > >>> Signed-off-by: John Ogness <john.ogness@linutronix.de> > >> Nice, it it better than v4. I am going to push this for linux-next. > >> > >> Reviewed-by: Petr Mladek <pmladek@suse.com> > > JFYI, I have just pushed this patch instead of the one > > from v4 into printk/linux.git, branch rework/kthreads. > > > > It means that this branch has been rebased. It will be > > used in the next refresh of linux-next. > > This patchset landed in linux next-20220426. In my tests I've found that > it causes deadlock on all my Amlogic Meson G12B/SM1 based boards: Odroid > C4/N2 and Khadas VIM3/VIM3l. The deadlock happens when system boots to > userspace and getty (with automated login) is executed. I even see the > bash prompt, but then the console is freezed. Reverting this patch > (e00cc0e1cbf4) on top of linux-next (together with 6b3d71e87892 to make > revert clean) fixes the issue.
Thanks a lot for the report!
Just by chance, do you have the log from the dead-locked boot stored in userspace and can you share it? I mean the log stored in /var/log/dmesg or journaltctl.
In the worst case, it might help to see log from the boot with the reverted patch. I would help us to see the ordering of various console-related operations on your system.
And regarding the console. Is it the graphics console (ttyX) or a serial one (ttyS) or yet another one?
Best Regards, Petr
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