Details u have to look at the source code, my guess is based on the following posting:
http://kerneltrap.org/mailarchive/linux-kernel/2008/1/23/595569 https://lkml.org/lkml/2011/6/21/249 logic: a. u want to be able to do printk from anywhere. b. but every call to printk requires a console_sem lock to be acquired. after acquiring console_sem, printk actually serializes the output to a memory buffer. c. now problem arises when printk is happening very fast, and so this type of locks is ill-suited for printk(). d. later than this patch is another attempt: https://patchwork.kernel.org/patch/1760211/ https://lkml.org/lkml/2012/10/20/90 where lazy irq work is being used instead. read through the comments in the intro to the patch - it covers a lot more than i mentioned here. In Documentation/lockdep_design.txt discuss about using irq tracing to trace the lock dependencies. Lock inversion is a common computer science problem....look up wiki. On Sat, Feb 9, 2013 at 10:55 AM, buyitian <[email protected]> wrote: > in the patch 0b5e1c5255e7ee8670e077e8224e5c2281229a5b, it releases > console_sem after logbuf_lock, the description of this patch is as below: > > Release console_sem after unlocking the logbuf_lock so that we don't > generate wakeups while holding logbuf_lock. This avoids some lock > inversion troubles once we remove the lockdep_off bits between > logbuf_lock and rq->lock (prints while holding rq->lock vs doing > wakeups while holding logbuf_lock). > There's of course still an actual deadlock where the printk()s under > rq->lock will issue a wakeup from the up() call, but lockdep won't > warn about that since semaphores are not tracked. > > could you please give me a detail example about the issue it tries > to fix? thanks. > > _______________________________________________ > Kernelnewbies mailing list > [email protected] > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies > > -- Regards, Peter Teoh
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