From: Michael Ellerman mpe@ellerman.id.au
commit 7c6986ade69e3c81bac831645bc72109cd798a80 upstream.
In raise_backtrace_ipi() we iterate through the cpumask of CPUs, sending each an IPI asking them to do a backtrace, but we don't wait for the backtrace to happen.
We then iterate through the CPU mask again, and if any CPU hasn't done the backtrace and cleared itself from the mask, we print a trace on its behalf, noting that the trace may be "stale".
This works well enough when a CPU is not responding, because in that case it doesn't receive the IPI and the sending CPU is left to print the trace. But when all CPUs are responding we are left with a race between the sending and receiving CPUs, if the sending CPU wins the race then it will erroneously print a trace.
This leads to spurious "stale" traces from the sending CPU, which can then be interleaved messily with the receiving CPU, note the CPU numbers, eg:
[ 1658.929157][ C7] rcu: Stack dump where RCU GP kthread last ran: [ 1658.929223][ C7] Sending NMI from CPU 7 to CPUs 1: [ 1658.929303][ C1] NMI backtrace for cpu 1 [ 1658.929303][ C7] CPU 1 didn't respond to backtrace IPI, inspecting paca. [ 1658.929362][ C1] CPU: 1 PID: 325 Comm: kworker/1:1H Tainted: G W E 5.13.0-rc2+ #46 [ 1658.929405][ C7] irq_soft_mask: 0x01 in_mce: 0 in_nmi: 0 current: 325 (kworker/1:1H) [ 1658.929465][ C1] Workqueue: events_highpri test_work_fn [test_lockup] [ 1658.929549][ C7] Back trace of paca->saved_r1 (0xc0000000057fb400) (possibly stale): [ 1658.929592][ C1] NIP: c00000000002cf50 LR: c008000000820178 CTR: c00000000002cfa0
To fix it, change the logic so that the sending CPU waits 5s for the receiving CPU to print its trace. If the receiving CPU prints its trace successfully then the sending CPU just continues, avoiding any spurious "stale" trace.
This has the added benefit of allowing all CPUs to print their traces in order and avoids any interleaving of their output.
Fixes: 5cc05910f26e ("powerpc/64s: Wire up arch_trigger_cpumask_backtrace()") Cc: stable@vger.kernel.org # v4.18+ Reported-by: Nathan Lynch nathanl@linux.ibm.com Signed-off-by: Michael Ellerman mpe@ellerman.id.au Link: https://lore.kernel.org/r/20210625140408.3351173-1-mpe@ellerman.id.au Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/powerpc/kernel/stacktrace.c | 27 +++++++++++++++++++++------ 1 file changed, 21 insertions(+), 6 deletions(-)
--- a/arch/powerpc/kernel/stacktrace.c +++ b/arch/powerpc/kernel/stacktrace.c @@ -19,6 +19,7 @@ #include <asm/ptrace.h> #include <asm/processor.h> #include <linux/ftrace.h> +#include <linux/delay.h> #include <asm/kprobes.h>
#include <asm/paca.h> @@ -204,17 +205,31 @@ static void handle_backtrace_ipi(struct
static void raise_backtrace_ipi(cpumask_t *mask) { + struct paca_struct *p; unsigned int cpu; + u64 delay_us;
for_each_cpu(cpu, mask) { - if (cpu == smp_processor_id()) + if (cpu == smp_processor_id()) { handle_backtrace_ipi(NULL); - else - smp_send_safe_nmi_ipi(cpu, handle_backtrace_ipi, 5 * USEC_PER_SEC); - } + continue; + }
- for_each_cpu(cpu, mask) { - struct paca_struct *p = paca_ptrs[cpu]; + delay_us = 5 * USEC_PER_SEC; + + if (smp_send_safe_nmi_ipi(cpu, handle_backtrace_ipi, delay_us)) { + // Now wait up to 5s for the other CPU to do its backtrace + while (cpumask_test_cpu(cpu, mask) && delay_us) { + udelay(1); + delay_us--; + } + + // Other CPU cleared itself from the mask + if (delay_us) + continue; + } + + p = paca_ptrs[cpu];
cpumask_clear_cpu(cpu, mask);