On Mon 2018-11-26 13:57:09, Sergey Senozhatsky wrote: > On (11/23/18 12:48), Petr Mladek wrote: > [..] > > > This should make serial consoles re-entrant. > > > So printk->console_driver_write() hopefully will not deadlock. > > > > Is the re-entrance safe? Some risk might be acceptable in Oops/panic > > situations. It is much less acceptable for random warnings. > > Good question. > > But what's the alternative? A deadlock in a serial console driver; such > that even panic() is not guaranteed to make through it (at least of now). > debug objects are used from the code which cannot re-entrant console > drivers. > > bust_spinlock is called from various paths, not only panic. > git grep bust_spinlocks | wc -l > 62 bust_spinlocks() is followed by die() in several situations. The rests seems to be Oops situations where we an invalid address is being accessed. There is a nontrivial chance that the system would die anyway. Now, if I look into Documentation/core-api/debug-objects.rst, the API is used to detect: - Activation of uninitialized objects - Initialization of active objects - Usage of freed/destroyed objects Of course, all the above situations might lead to the system crash. But even in the worst case, use-after-free, there is a non-trivial chance that the data still would be valid and the system would survive. There might be many other warnings of the same severity. > So we already switch to re-entrant consoles (and accept the risks) in > mm/fault.c, kernel/traps.c and so on. Which, I guess, makes us a little > more confident, faults/traps happen often enough. Where is the border line, please? Do we want to have the kernel sources full of bust_spinlocks() callers? > It seems, that, more or less, serial consoles are ready to handle it. > UART consoles in ->write() callbacks just do a bunch of writel() [for > every char + \r\n]. But oops_in_progress does not affect only serial (UART) consoles. We want safe lockless consoles. We do not want to run a most-of-the-time-safe code too often. BTW: I have heard that someone from the RT people is working on a big printk() rewrite. One part is a lockless buffer. Another part should be a different handling of safe (lockless) and more complicated consoles. It was presented on some recent conference (I forgot which one). I do not know any details. But the first version should be sent in a near future. Best Regards, Petr
