From caffd39d4f15810c653fa8686aaf43c11c18d854 Mon Sep 17 00:00:00 2001 From: Sergey Senozhatsky Date: Tue, 3 Mar 2020 20:30:02 +0900 Subject: printk: queue wake_up_klogd irq_work only if per-CPU areas are ready commit ab6f762f0f53162d41497708b33c9a3236d3609e upstream. printk_deferred(), similarly to printk_safe/printk_nmi, does not immediately attempt to print a new message on the consoles, avoiding calls into non-reentrant kernel paths, e.g. scheduler or timekeeping, which potentially can deadlock the system. Those printk() flavors, instead, rely on per-CPU flush irq_work to print messages from safer contexts. For same reasons (recursive scheduler or timekeeping calls) printk() uses per-CPU irq_work in order to wake up user space syslog/kmsg readers. However, only printk_safe/printk_nmi do make sure that per-CPU areas have been initialised and that it's safe to modify per-CPU irq_work. This means that, for instance, should printk_deferred() be invoked "too early", that is before per-CPU areas are initialised, printk_deferred() will perform illegal per-CPU access. Lech Perczak [0] reports that after commit 1b710b1b10ef ("char/random: silence a lockdep splat with printk()") user-space syslog/kmsg readers are not able to read new kernel messages. The reason is printk_deferred() being called too early (as was pointed out by Petr and John). Fix printk_deferred() and do not queue per-CPU irq_work before per-CPU areas are initialized. Link: https://lore.kernel.org/lkml/aa0732c6-5c4e-8a8b-a1c1-75ebe3dca05b@camlintechnologies.com/ Reported-by: Lech Perczak Signed-off-by: Sergey Senozhatsky Tested-by: Jann Horn Reviewed-by: Petr Mladek Cc: Greg Kroah-Hartman Cc: Theodore Ts'o Cc: John Ogness Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- init/main.c | 1 - 1 file changed, 1 deletion(-) (limited to 'init') diff --git a/init/main.c b/init/main.c index ec78f2312610..fdfef08da0c4 100644 --- a/init/main.c +++ b/init/main.c @@ -641,7 +641,6 @@ asmlinkage __visible void __init start_kernel(void) softirq_init(); timekeeping_init(); time_init(); - printk_safe_init(); perf_event_init(); profile_init(); call_function_init(); -- cgit v1.2.3