Re: [PATCH 1/2] printk: Introduce LOUD_CON flag

[John Ogness <john.ogness@linutronix.de>]

On 2024-10-17, Petr Mladek <pmladek@suse.com> wrote:
> # echo h >/proc/sysrq-trigger
>
> produced:
>
> [   53.669907] BUG: assuming non migratable context at kernel/printk/printk_safe.c:23
> [   53.669920] in_atomic(): 0, irqs_disabled(): 0, migration_disabled() 0 pid: 1637, name: bash
> [   53.669931] 2 locks held by bash/1637:
> [   53.669936]  #0: ffff8ae680a384a8 (sb_writers#4){.+.+}-{0:0}, at: ksys_write+0x6e/0xf0
> [   53.669968]  #1: ffffffff83f226e0 (rcu_read_lock){....}-{1:3}, at: __handle_sysrq+0x3d/0x120
> [   53.670002] CPU: 2 UID: 0 PID: 1637 Comm: bash Not tainted 6.12.0-rc3-default+ #67
> [   53.670011] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.3-2-gc13ff2cd-prebuilt.qemu.org 04/01/2014
> [   53.670020] Call Trace:
> [   53.670026]  <TASK>
> [   53.670045]  dump_stack_lvl+0x6c/0xa0
> [   53.670064]  __cant_migrate.cold+0x7c/0x89
> [   53.670080]  printk_loud_console_enter+0x15/0x30
> [   53.670088]  __handle_sysrq+0x60/0x120
> [   53.670104]  write_sysrq_trigger+0x6a/0xa0
> [   53.670120]  proc_reg_write+0x5f/0xb0
> [   53.670132]  vfs_write+0xf9/0x540
> [   53.670147]  ? __lock_release.isra.0+0x1a6/0x2c0
> [   53.670172]  ? do_user_addr_fault+0x38c/0x720
> [   53.670197]  ksys_write+0x6e/0xf0
> [   53.670220]  do_syscall_64+0x79/0x190
> [   53.670238]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
>
> IMHO, the best solution would be to call migrate_disable()/enable()
> in printk_loud_console_enter()/exit().

That will not work because migrate_enable() can only be called from
can_sleep context. Instead, the migrate_disable()/enable() should be at
the few (one?) call sites where printk_loud_console_enter()/exit() is
used from task context.

This is also how printk_deferred_enter()/exit() works, relying on the
caller to disable migration if necessary.

John