Re: [PATCH v3] tty/sysrq: replace smp_processor_id() with get_cpu()

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On 18. 09. 23, 10:10, 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@xxxxxxxxxxxxxxx
Fixes: 47cab6a722d4 ("debug lockups: Improve lockup detection, fix generic arch fallback")
Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
---
Changes since v2:
- Add changelog and resend

Changes since v1:
- Add "Cc: stable@xxxxxxxxxxxxxxx" 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());

Why not call put_cpu() right here?

  		if (regs)
  			show_regs(regs);
  		else
  			show_stack(NULL, NULL, KERN_INFO);
schedule_work(&sysrq_showallcpus);
+		put_cpu();

Why wait so long here after you have scheduled work?  Please drop the
cpu reference right away, you don't need to hold it for this length of
time, right?

As I understand it, this way, schedule_work() will queue the work on the "gotten" (current) CPU. So sysrq_showregs_othercpus() will really dump other than the "gotten" cpu.

If that is the case, it indeed should have been described in the commit log.

regards,
--
js
suse labs




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