Messages in this thread | | | From | Konstantin Khlebnikov <> | Date | Sat, 13 Jul 2019 17:03:14 +0300 | Subject | Re: [PATCH] kernel/printk: prevent deadlock at calling kmsg_dump from NMI context |
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On Sat, Jul 13, 2019 at 4:20 PM Sergey Senozhatsky <sergey.senozhatsky@gmail.com> wrote: > > On (07/13/19 09:46), Konstantin Khlebnikov wrote: > > > On (07/12/19 17:54), Konstantin Khlebnikov wrote: > > > > Yep printk() can deal with NMI, but kmsg_dump() is a different beast. > > It reads printk buffer and saves content into persistent storage like ACPI ERST. > > Ah, sorry! I misread your patch. Yeah, I see what you are doing. > > OK. So, I guess that for kmsg_dump(KMSG_DUMP_PANIC) we should be > fine in general. > > We call kmsg_dump(KMSG_DUMP_PANIC) after smp_send_stop() and after > printk_safe_flush_on_panic(). printk_safe_flush_on_panic() resets > the state of logbuf_lock, so logbuf_lock, in general case, should > be unlocked by the time we call kmsg_dump(KMSG_DUMP_PANIC). > Even for nested contexts. > > CPU0 > printk() > logbuf_lock_irqsave(flags) > -> NMI > panic() > smp_send_stop() > printk_safe_flush_on_panic() > raw_spin_lock_init(&logbuf_lock) << reinit >> > kmsg_dump(KMSG_DUMP_PANIC) > logbuf_lock_irqsave(flags) << expected to be OK >> > > So do we have strong reasons to disable NMI->panic->kmsg_dump(DUMP_PANIC)? > > Other kmsg_dump(), maybe, can experience some troubles sometimes, > need to check that.
Indeed, panic is especially handled and looks fine.
Sanity check in my patch could be relaxed:
if (WARN_ON_ONCE(reason != KMSG_DUMP_PANIC && in_nmi())) return;
> > -ss
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