Messages in this thread | | | From | Linus Torvalds <> | Date | Wed, 5 Jul 2017 11:23:47 -0700 | Subject | Re: [GIT PULL] printk for 4.13 |
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On Mon, Jul 3, 2017 at 7:24 AM, Petr Mladek <pmladek@suse.com> wrote: > > - Store printk() messages into the main log buffer directly even in NMI > when the lock is available. It is the best effort to print even large > chunk of text. It is handy, for example, when all ftrace messages > are printed during the system panic in NMI.
No, this is entirely buggered.
You can't just do
raw_spin_is_locked(&logbuf_lock)
to test whether you can get the logbuf_lock and then call vprintk_default().
It's not just about deadlock avoidance, the code will call things like down_trylock() on the console semaphore, and that operation is fundamentally not NMI-safe. The semaphore count is literally protected by a irq-safe (BUT NOT NMI-SAFE!) semaphore spinlock.
So now you can instead deadlock just on the internal console semaphore spinlock (ie somebody is doing "console_lock()", NMI comes in, *BOOM*.
Maybe I'm missing something, but I'm skipping this pull request as "completely broken".
Linus
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