Messages in this thread | | | Date | Thu, 10 Feb 2022 10:22:05 +0100 | From | Petr Mladek <> | Subject | Re: [PATCH v4 0/4] printk: reduce deadlocks during panic |
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On Wed 2022-02-02 09:18:17, Stephen Brennan wrote: > When a caller writes heavily to the kernel log (e.g. writing to > /dev/kmsg in a loop) while another panics, there's currently a high > likelihood of a deadlock (see patch 2 for the full description of this > deadlock). > > The principle fix is to disable the optimistic spin once panic_cpu is > set, so the panic CPU doesn't spin waiting for a halted CPU to hand over > the console_sem. > > However, this exposed us to a livelock situation, where the panic CPU > holds the console_sem, and another CPU could fill up the log buffer > faster than the consoles could drain it, preventing the panic from > progressing and halting the other CPUs. To avoid this, patch 3 adds a > mechanism to suppress printk (from non-panic-CPU) during panic, if we > reach a threshold of dropped messages. > > A major goal with all of these patches is to try to decrease the > likelihood that another CPU is holding the console_sem when we halt it > in panic(). This reduces the odds of needing to break locks and > potentially encountering further deadlocks with the console drivers. > > To test, I use the following script, kmsg_panic.sh: > > #!/bin/bash > date > # 991 chars (based on log buffer size): > chars="$(printf 'a%.0s' {1..991})" > while :; do > echo $chars > /dev/kmsg > done & > echo c > /proc/sysrq-trigger & > date > exit > > I defined a hang as any time the system did not reboot to a login prompt > on the serial console within 60 seconds. Here are the statistics on > hangs using this script, before and after the patch. > > before: 776 hangs / 1484 trials - 52.3% > after : 0 hangs / 15k trials - 0.0% > > Stephen Brennan (4): > printk: Add panic_in_progress helper > printk: disable optimistic spin during panic > printk: Avoid livelock with heavy printk during panic > printk: Drop console_sem during panic > > kernel/printk/printk.c | 55 +++++++++++++++++++++++++++++++++++++++++- > 1 file changed, 54 insertions(+), 1 deletion(-)
For the entire patchset:
Reviewed-by: Petr Mladek <pmladek@suse.com>
It looks ready for linux-next from my POV. I am going to push it early next week unless anyone complains in the meantime.
Best Regards, Petr Mladek
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