Messages in this thread | | | Date | Wed, 30 Nov 2022 19:12:27 +0100 | From | Petr Mladek <> | Subject | Re: [linux-next:master] [printk] 8fc5f5fc7f: WARNING:at_kernel/printk/printk.c:#console_flush_all |
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On Thu 2022-12-01 00:57:06, kernel test robot wrote: > Greeting, > > FYI, we noticed WARNING:at_kernel/printk/printk.c:#console_flush_all due to commit (built with clang-14): > > commit: 8fc5f5fc7f52a733fcc8b3939d172b9248e63871 ("printk: introduce console_list_lock") > https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git master > > in testcase: boot > > on test machine: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -smp 2 -m 16G > > caused below changes (please refer to attached dmesg/kmsg for entire log/backtrace): > > > [ 0.000000][ T0] ------------[ cut here ]------------ > [ 0.000000][ T0] WARNING: CPU: 0 PID: 0 at kernel/printk/printk.c:116 console_flush_all (printk.c:?)
When I checked out the commit 8fc5f5fc7f52a733fcc8b3939d172b9248e63871 ("printk: introduce console_list_lock") then kernel/printk/printk.c:116 is:
114 void lockdep_assert_console_list_lock_held(void) 115 { 116 lockdep_assert_held(&console_mutex); 117 } 118 EXPORT_SYMBOL(lockdep_assert_console_list_lock_held);
It is used in
#define for_each_console(con) \ lockdep_assert_console_list_lock_held(); \ hlist_for_each_entry(con, &console_list, node)
that is used in
static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handover) { [...] for_each_console(con) {
that is called without console_list_lock().
Hmm, we could not take console_list_lock() here. It would violate lock ordering. console_flush_all() is called under console_lock(). And console_list_lock() is taken outside of console_lock() in register_console().
Fortunately, we do not have to do it. for_each_console() is still safe under console_lock() at this stage. And later patches will switch it to for_each_console_srcu() that will not require console_list_lock().
So, the solution is to remove the assert in for_each_console() in the commit 8fc5f5fc7f52a733fcc8b3939d172b9248e63871 ("printk: introduce console_list_lock").
We need to add the assert later when removing the console_lock() synchronization of @console_list in register_console().
Best Regards, Petr
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