Commit ab6f762f authored by Sergey Senozhatsky's avatar Sergey Senozhatsky Committed by Linus Torvalds

printk: queue wake_up_klogd irq_work only if per-CPU areas are ready

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 1b710b1b ("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: default avatarLech Perczak <l.perczak@camlintechnologies.com>
Signed-off-by: default avatarSergey Senozhatsky <sergey.senozhatsky@gmail.com>
Tested-by: default avatarJann Horn <jannh@google.com>
Reviewed-by: default avatarPetr Mladek <pmladek@suse.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: John Ogness <john.ogness@linutronix.de>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 87ad46e6
......@@ -202,7 +202,6 @@ __printf(1, 2) void dump_stack_set_arch_desc(const char *fmt, ...);
void dump_stack_print_info(const char *log_lvl);
void show_regs_print_info(const char *log_lvl);
extern asmlinkage void dump_stack(void) __cold;
extern void printk_safe_init(void);
extern void printk_safe_flush(void);
extern void printk_safe_flush_on_panic(void);
#else
......@@ -269,10 +268,6 @@ static inline void dump_stack(void)
{
}
static inline void printk_safe_init(void)
{
}
static inline void printk_safe_flush(void)
{
}
......
......@@ -913,7 +913,6 @@ asmlinkage __visible void __init start_kernel(void)
boot_init_stack_canary();
time_init();
printk_safe_init();
perf_event_init();
profile_init();
call_function_init();
......
......@@ -23,6 +23,9 @@ __printf(1, 0) int vprintk_func(const char *fmt, va_list args);
void __printk_safe_enter(void);
void __printk_safe_exit(void);
void printk_safe_init(void);
bool printk_percpu_data_ready(void);
#define printk_safe_enter_irqsave(flags) \
do { \
local_irq_save(flags); \
......@@ -64,4 +67,6 @@ __printf(1, 0) int vprintk_func(const char *fmt, va_list args) { return 0; }
#define printk_safe_enter_irq() local_irq_disable()
#define printk_safe_exit_irq() local_irq_enable()
static inline void printk_safe_init(void) { }
static inline bool printk_percpu_data_ready(void) { return false; }
#endif /* CONFIG_PRINTK */
......@@ -460,6 +460,18 @@ static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
static char *log_buf = __log_buf;
static u32 log_buf_len = __LOG_BUF_LEN;
/*
* We cannot access per-CPU data (e.g. per-CPU flush irq_work) before
* per_cpu_areas are initialised. This variable is set to true when
* it's safe to access per-CPU data.
*/
static bool __printk_percpu_data_ready __read_mostly;
bool printk_percpu_data_ready(void)
{
return __printk_percpu_data_ready;
}
/* Return log buffer address */
char *log_buf_addr_get(void)
{
......@@ -1146,12 +1158,28 @@ static void __init log_buf_add_cpu(void)
static inline void log_buf_add_cpu(void) {}
#endif /* CONFIG_SMP */
static void __init set_percpu_data_ready(void)
{
printk_safe_init();
/* Make sure we set this flag only after printk_safe() init is done */
barrier();
__printk_percpu_data_ready = true;
}
void __init setup_log_buf(int early)
{
unsigned long flags;
char *new_log_buf;
unsigned int free;
/*
* Some archs call setup_log_buf() multiple times - first is very
* early, e.g. from setup_arch(), and second - when percpu_areas
* are initialised.
*/
if (!early)
set_percpu_data_ready();
if (log_buf != __log_buf)
return;
......@@ -2975,6 +3003,9 @@ static DEFINE_PER_CPU(struct irq_work, wake_up_klogd_work) = {
void wake_up_klogd(void)
{
if (!printk_percpu_data_ready())
return;
preempt_disable();
if (waitqueue_active(&log_wait)) {
this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP);
......@@ -2985,6 +3016,9 @@ void wake_up_klogd(void)
void defer_console_output(void)
{
if (!printk_percpu_data_ready())
return;
preempt_disable();
__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
......
......@@ -27,7 +27,6 @@
* There are situations when we want to make sure that all buffers
* were handled or when IRQs are blocked.
*/
static int printk_safe_irq_ready __read_mostly;
#define SAFE_LOG_BUF_LEN ((1 << CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT) - \
sizeof(atomic_t) - \
......@@ -51,7 +50,7 @@ static DEFINE_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq);
/* Get flushed in a more safe context. */
static void queue_flush_work(struct printk_safe_seq_buf *s)
{
if (printk_safe_irq_ready)
if (printk_percpu_data_ready())
irq_work_queue(&s->work);
}
......@@ -402,14 +401,6 @@ void __init printk_safe_init(void)
#endif
}
/*
* In the highly unlikely event that a NMI were to trigger at
* this moment. Make sure IRQ work is set up before this
* variable is set.
*/
barrier();
printk_safe_irq_ready = 1;
/* Flush pending messages that did not have scheduled IRQ works. */
printk_safe_flush();
}
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