On 8/23/23 4:06 PM, Muhammad Usama Anjum wrote:
On 8/22/23 6:24 PM, Greg Kroah-Hartman wrote:
On Tue, Aug 22, 2023 at 03:26:06PM +0500, Muhammad Usama Anjum wrote:
The smp_processor_id() shouldn't be called from preemptible code. Instead use get_cpu() and put_cpu() which disables preemption in addition to getting the processor id. This fixes the following bug:
[ 119.143590] sysrq: Show backtrace of all active CPUs [ 119.143902] BUG: using smp_processor_id() in preemptible [00000000] code: bash/873 [ 119.144586] caller is debug_smp_processor_id+0x20/0x30 [ 119.144827] CPU: 6 PID: 873 Comm: bash Not tainted 5.10.124-dirty #3 [ 119.144861] Hardware name: QEMU QEMU Virtual Machine, BIOS 2023.05-1 07/22/2023 [ 119.145053] Call trace: [ 119.145093] dump_backtrace+0x0/0x1a0 [ 119.145122] show_stack+0x18/0x70 [ 119.145141] dump_stack+0xc4/0x11c [ 119.145159] check_preemption_disabled+0x100/0x110 [ 119.145175] debug_smp_processor_id+0x20/0x30 [ 119.145195] sysrq_handle_showallcpus+0x20/0xc0 [ 119.145211] __handle_sysrq+0x8c/0x1a0 [ 119.145227] write_sysrq_trigger+0x94/0x12c [ 119.145247] proc_reg_write+0xa8/0xe4 [ 119.145266] vfs_write+0xec/0x280 [ 119.145282] ksys_write+0x6c/0x100 [ 119.145298] __arm64_sys_write+0x20/0x30 [ 119.145315] el0_svc_common.constprop.0+0x78/0x1e4 [ 119.145332] do_el0_svc+0x24/0x8c [ 119.145348] el0_svc+0x10/0x20 [ 119.145364] el0_sync_handler+0x134/0x140 [ 119.145381] el0_sync+0x180/0x1c0
Cc: stable@vger.kernel.org Fixes: 47cab6a722d4 ("debug lockups: Improve lockup detection, fix generic arch fallback")This commit had introduced the smp_processor_id() function in
sysrq_handle_showallcpus().
How has this never shown up before now? What changed to cause this to now be triggered? This feels odd that no one has seen this in the past 20+ years :(
Not sure. Probably the combination of reproduction has happened now. The following three conditions are needed for the warning to appear:
- Enable CONFIG_DEBUG_PREEMPT
- Arch which doesn't define arch_trigger_all_cpu_backtrace such as arm64
- Trigger showallcpu's stack sysrq
Any thoughts about the patch?
Signed-off-by: Muhammad Usama Anjum usama.anjum@collabora.com
Changes since v2:
- Add changelog and resend
Changes since v1:
- Add "Cc: stable@vger.kernel.org" tag
drivers/tty/sysrq.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c index 23198e3f1461a..6b4a28bcf2f5f 100644 --- a/drivers/tty/sysrq.c +++ b/drivers/tty/sysrq.c @@ -262,13 +262,14 @@ static void sysrq_handle_showallcpus(u8 key) if (in_hardirq()) regs = get_irq_regs();
pr_info("CPU%d:\n", smp_processor_id());
pr_info("CPU%d:\n", get_cpu());schedule_work(&sysrq_showallcpus);
put_cpu();Why are you putting the cpu _after_ you schedule the work?
The sysrq_showallcpus work prints stack traces on all CPUs other than the current CPU. So we are re-enabling preemption after scheduling work from current CPU. So that it doesn't get changed.
thanks,
greg k-h