On Thu, Apr 30, 2020 at 09:59:09AM -0700, Douglas Anderson wrote: > The original implementation of kgdboc_earlycon included a comment > about how it was impossible to get notified about the boot console > going away without making changes to the Linux core. Since folks > often don't want to change the Linux core for kgdb's purposes, the > kgdboc_earlycon implementation did a bit of polling to figure out when > the boot console went away. > > It turns out, though, that it is possible to get notified about the > boot console going away. The solution is either clever or a hack > depending on your viewpoint. ...or, perhaps, a clever hack. All we > need to do is head-patch the "exit" routine of the boot console. We > know that "struct console" must be writable because it has a "next" > pointer in it, so we can just put our own exit routine in, do our > stuff, and then call back to the original. I think I'm in the hack camp on this one! > This works great to get notified about the boot console going away. > The (slight) problem is that in the context of the boot console's exit > routine we can't call tty_find_polling_driver(). I presume this is something to do with the tty_mutex? > We solve this by > kicking off some work on the system_wq when we get notified and this > works pretty well. There are some problems with the workqueue approach. Firstly, its runs too early on many systems (esp. those that register the console from a console initcall. kgdboc cannot find the tty, I think because the console is registered a long time before the corresponding tty comes up. Secondly I am seeing warnings related to the tty_mutex where the might_sleep() machinery ends up flushing the active workqueue. [ 39.298332] ------------[ cut here ]------------ [ 39.298332] WARNING: CPU: 0 PID: 5 at kernel/workqueue.c:3033 __flush_work+00 [ 39.298332] Modules linked in: [ 39.298332] CPU: 0 PID: 5 Comm: kworker/0:0 Not tainted 5.7.0-rc3+ #47 [ 39.298332] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-204 [ 39.298332] Workqueue: events kgdboc_earlycon_exit_work_fn [ 39.298332] RIP: 0010:__flush_work+0x19c/0x1c0 [ 39.298332] Code: 4c 8b 6d 20 e9 06 ff ff ff 41 c6 04 24 00 fb 45 31 f6 eb 8f [ 39.298332] RSP: 0018:ffff993500033dd0 EFLAGS: 00010246 [ 39.298332] RAX: 0000000000000000 RBX: ffffffffadcd0720 RCX: 0000000000000001 [ 39.298332] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffffadcd0820 [ 39.298332] RBP: ffff8a633ec299c0 R08: 0000000000000000 R09: 0000000000000001 [ 39.298332] R10: 000000000000000a R11: f000000000000000 R12: 00000000ffffffed [ 39.298332] R13: ffff8a633e408840 R14: 0000000000000000 R15: ffff8a633e408840 [ 39.298332] FS: 0000000000000000(0000) GS:ffff8a633ec00000(0000) knlGS:00000 [ 39.298332] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 39.298332] CR2: ffff8a6333201000 CR3: 0000000032a0a000 CR4: 00000000000006f0 [ 39.298332] Call Trace: [ 39.298332] ? _cond_resched+0x10/0x20 [ 39.298332] ? mutex_lock+0x9/0x30 [ 39.298332] ? tty_find_polling_driver+0x134/0x1a0 [ 39.298332] configure_kgdboc+0x12d/0x1c0 [ 39.298332] kgdboc_earlycon_exit_work_fn+0x1a/0x40 [ 39.298332] process_one_work+0x1d3/0x380 [ 39.298332] worker_thread+0x45/0x3c0 [ 39.298332] kthread+0xf6/0x130 [ 39.298332] ? process_one_work+0x380/0x380 [ 39.298332] ? kthread_park+0x80/0x80 [ 39.298332] ret_from_fork+0x22/0x40 [ 39.298332] ---[ end trace 1190f578d6e11204 ]--- [ 39.298338] KGDB: Unregistered I/O driver kgdboc_earlycon, debugger disabled Daniel.
