Hi,

On 09-08-18 15:29, Daniel Vetter wrote:
On Thu, Aug 9, 2018 at 1:42 PM, Hans de Goede <hdego...@redhat.com> wrote:
Taking over the console involves allocating mem with GFP_KERNEL, talking
to drm drivers, etc. So this should not be done from an atomic context.

But the console-output trigger deferred console takeover may happen from an
atomic context, which leads to "BUG: sleeping function called from invalid
context" errors.

This commit fixes these errors by doing the deferred takeover from a
workqueue when the notifier runs from an atomic context.

Note this uses in_atomic, as checkpatch points out this should not be
done from driver code. But the console subsys is not really normal driver
code, specifically it plays some tricks where it disables some locking
when logging an oops, or when logging a lockdep bug when lockdep debugging
is turned on, so we need to make an exception here.

Signed-off-by: Hans de Goede <hdego...@redhat.com>
---
Changes in v2:
-Add a comment fbcon.c and the commit message about why we need to use
  in_atomic here, no functional changes
---
  drivers/video/fbdev/core/fbcon.c | 29 +++++++++++++++++++++++++++--
  1 file changed, 27 insertions(+), 2 deletions(-)

diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index ef8b2d0b7071..31f518f8dde7 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -3592,7 +3592,20 @@ static int fbcon_init_device(void)
  }

  #ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER
+static void fbcon_register_existing_fbs(struct work_struct *work)
+{
+       int i;
+
+       console_lock();
+
+       for_each_registered_fb(i)
+               fbcon_fb_registered(registered_fb[i]);
+
+       console_unlock();
+}
+
  static struct notifier_block fbcon_output_nb;
+static DECLARE_WORK(fbcon_deferred_takeover_work, fbcon_register_existing_fbs);

  static int fbcon_output_notifier(struct notifier_block *nb,
                                  unsigned long action, void *data)
@@ -3607,8 +3620,20 @@ static int fbcon_output_notifier(struct notifier_block 
*nb,
         deferred_takeover = false;
         logo_shown = FBCON_LOGO_DONTSHOW;

-       for_each_registered_fb(i)
-               fbcon_fb_registered(registered_fb[i]);
+       /*
+        * Normally all console output happens in a sleeping context, but
+        * during oopses the kernel goes into a special mode where the
+        * console code may not sleep. We check for this using in_atomic
+        * as the note about in_atomic in preempt.h mentions, in_atomic
+        * does not detect a spinlock being held when non-preemptible, so
+        * we also check for irqs_disabled which covers this case.
+        */

If this is only for oopses, then opps_in_progress is what you're
looking for.

Unfortunately that is not good enough as mentioned in the v2 commit
message, console output may also happen in atomic context when the
lockdep code has found an issue (from print_circular_bug).

I've tried replacing:

       if (in_atomic() || irqs_disabled()) {

With:

       if (oops_in_progress) {

On a system where I know the lockdep code will report a problem
wrt the bttv driver and here is what happened:

[    7.937690] fbcon: Taking over console
[    7.937691] BUG: sleeping function called from invalid context at 
mm/slab.h:421
[    7.937691] in_atomic(): 1, irqs_disabled(): 1, pid: 561, name: systemd-udevd
[    7.937692] INFO: lockdep is turned off.
[    7.937692] irq event stamp: 196513
[    7.937692] hardirqs last  enabled at (196513): [<ffffffffbba4314b>] 
_raw_spin_unlock_irqrestore+0x4b/0x60
[    7.937692] hardirqs last disabled at (196512): [<ffffffffbba43962>] 
_raw_spin_lock_irqsave+0x22/0x81
[    7.937693] softirqs last  enabled at (196504): [<ffffffffbbe0038c>] 
__do_softirq+0x38c/0x4f7
[    7.937693] softirqs last disabled at (196401): [<ffffffffbb0c19be>] 
irq_exit+0x10e/0x120
[    7.937694] CPU: 1 PID: 561 Comm: systemd-udevd Not tainted 
4.18.0-0.rc8.git1.1.hdg1.fc29.x86_64 #1
[    7.937694] Hardware name: To Be Filled By O.E.M. To Be Filled By 
O.E.M./B150M Pro4S/D3, BIOS P7.10 12/06/2016
[    7.937694] Call Trace:
[    7.937694]  dump_stack+0x85/0xc0
[    7.937695]  ___might_sleep.cold.72+0xac/0xbc
[    7.937695]  kmem_cache_alloc_trace+0x202/0x2f0
[    7.937695]  ? fbcon_startup+0xae/0x300
[    7.937695]  fbcon_startup+0xae/0x300
[    7.937696]  do_take_over_console+0x6d/0x180
[    7.937696]  do_fbcon_takeover+0x58/0xb0
[    7.937696]  fbcon_output_notifier.cold.35+0x18/0x44
[    7.937697]  notifier_call_chain+0x39/0x90
[    7.937697]  vt_console_print+0x363/0x420
[    7.937697]  console_unlock+0x422/0x610
[    7.937697]  vprintk_emit+0x268/0x540
[    7.937698]  ? kernel_text_address+0xe5/0xf0
[    7.937698]  printk+0x58/0x6f
[    7.937698]  print_circular_bug_header.cold.56+0x17/0x9c
[    7.937698]  print_circular_bug.isra.38+0x7c/0xb0
[    7.937699]  __lock_acquire+0x139a/0x16c0
[    7.937699]  lock_acquire+0x9e/0x1b0
[    7.937699]  ? find_ref_lock+0x21/0x40 [videodev]
[    7.937699]  ? find_ref_lock+0x21/0x40 [videodev]
[    7.937700]  __mutex_lock+0x88/0xa10
[    7.937700]  ? find_ref_lock+0x21/0x40 [videodev]
[    7.937700]  ? bttv_gpio_bits+0x22/0x60 [bttv]
[    7.937700]  ? native_sched_clock+0x3e/0xa0
[    7.937701]  ? mark_held_locks+0x57/0x80
[    7.937701]  ? _raw_spin_unlock_irqrestore+0x4b/0x60
[    7.937701]  ? find_ref_lock+0x21/0x40 [videodev]
[    7.937701]  find_ref_lock+0x21/0x40 [videodev]
[    7.937702]  v4l2_ctrl_find+0xa/0x20 [videodev]
[    7.937702]  audio_mute+0x38/0x120 [bttv]
[    7.937702]  bttv_s_ctrl+0x2d5/0x3b0 [bttv]
[    7.937702]  __v4l2_ctrl_handler_setup+0xdf/0x130 [videodev]
[    7.937703]  v4l2_ctrl_handler_setup+0x27/0x40 [videodev]
[    7.937703]  bttv_probe.cold.45+0x8f4/0xca7 [bttv]
[    7.937703]  local_pci_probe+0x41/0x90
[    7.937704]  pci_device_probe+0x118/0x1a0
[    7.937704]  driver_probe_device+0x2da/0x450
[    7.937704]  __driver_attach+0xe1/0x110
[    7.937704]  ? driver_probe_device+0x450/0x450
[    7.937705]  ? driver_probe_device+0x450/0x450
[    7.937705]  bus_for_each_dev+0x79/0xc0
[    7.937705]  ? deferred_probe_initcall+0x30/0x30
[    7.937705]  bus_add_driver+0x155/0x230
[    7.937706]  ? 0xffffffffc07a4000
[    7.937706]  driver_register+0x6b/0xb0
[    7.937706]  ? 0xffffffffc07a4000
[    7.937706]  bttv_init_module+0xcd/0xe3 [bttv]
[    7.937706]  do_one_initcall+0x5d/0x362
[    7.937707]  ? rcu_read_lock_sched_held+0x6b/0x80
[    7.937707]  ? kmem_cache_alloc_trace+0x293/0x2f0
[    7.937707]  ? do_init_module+0x22/0x210
[    7.937707]  do_init_module+0x5a/0x210
[    7.937708]  load_module+0x21e6/0x2610
[    7.937708]  ? ima_post_read_file+0x10c/0x119
[    7.937708]  ? __do_sys_finit_module+0xad/0x110
[    7.937708]  __do_sys_finit_module+0xad/0x110
[    7.937709]  do_syscall_64+0x60/0x1f0
[    7.937709]  entry_SYSCALL_64_after_hwframe+0x49/0xbe

Follow by a printing of the actual lockdep issue.

This is what I wrote this patch for in the first place
and checking for oops_in_progress does not fix this case.

So I believe we need to keep using the:

       if (in_atomic() || irqs_disabled()) {

Check, Bartlomiej can you apply v2 please, or let me know if
you want any other changes?

At least that's what we've switched to in drm_fb_helper.c
instead of a cargo-culted pile of in_atimc/irq/kgdb checks. And since
the box will die right afterwards, you might as well just bail out
directly and not bother with scheduling the worker?

Well with a lockdep bug the box won't die, depending on the actual
locking issue it may be quite harmless.

Regards,

Hans



Again, that's what
we've been doing in the drm fbdev emulation code.
-Daniel

+       if (in_atomic() || irqs_disabled()) {
+               schedule_work(&fbcon_deferred_takeover_work);
+       } else {
+               for_each_registered_fb(i)
+                       fbcon_fb_registered(registered_fb[i]);
+       }

         return NOTIFY_OK;
  }
--
2.18.0

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