Messages in this thread | | | Subject | Re: [PATCH v2 1/1] powerpc/crash: Use NMI context for printk when starting to crash | From | Leonardo Bras <> | Date | Wed, 13 May 2020 02:16:58 -0300 |
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Hello Nick, thanks for your feedback. Comments inline:
On Wed, 2020-05-13 at 14:36 +1000, Nicholas Piggin wrote: > Excerpts from Leonardo Bras's message of May 13, 2020 7:45 am: > > Currently, if printk lock (logbuf_lock) is held by other thread during > > crash, there is a chance of deadlocking the crash on next printk, and > > blocking a possibly desired kdump. > > > > At the start of default_machine_crash_shutdown, make printk enter > > NMI context, as it will use per-cpu buffers to store the message, > > and avoid locking logbuf_lock. > > printk_nmi_enter is used in one other place outside nmi_enter. > > Is there a different/better way to handle this? What do other > architectures do?
To be honest, I was unaware of nmi_enter() and I have yet to study what other architectures do here.
> Other subsystems get put into an nmi-mode when we call nmi_enter > (lockdep, ftrace, rcu etc). It seems like those would be useful for > similar reasons, so at least explaining why that is not used in a > comment would be good.
My reasoning for using printk_nmi_enter() here was only to keep it from using printk regular buffer (and locking logbuf_lock) at this point of the crash.
I have yet to see how nmi_enter() extra functions would happen to interfere with the crash at this point.
(In a quick look at x86, (native_machine_crash_shutdown) I could not see it using any printk, so it may not be necessary).
> Aside from that, I welcome any effort to make our crashes more reliable > so thanks for working on this stuff. > > Thanks, > Nick
Thank you, it means a lot.
Leonardo Bras
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