If we end up executing vprintk_alt() then we have a printk recursion. Set alt_printk_ctx `ALT_PRINTK_RECURSION_MASK' bit in vprintk_alt() to indicate that recutsion and report the "BUG: recent printk recursion!" problem later from __alt_printk_flush().
Example: BUG: recent printk recursion! ------------[ cut here ]------------ WARNING: CPU: 3 PID: 366 at kernel/printk/printk.c:1803 vprintk_emit+0x139/0x38c CPU: 3 PID: 366 Comm: bash Call Trace: [<ffffffff811be508>] dump_stack+0x4d/0x63 [<ffffffff81039932>] __warn+0xb8/0xd3 [<ffffffff810399b3>] warn_slowpath_null+0x18/0x1a [<ffffffff8106bfdb>] vprintk_emit+0x139/0x38c [<ffffffff8106c390>] vprintk_default+0x18/0x1a [<ffffffff8106d1e6>] vprintk_func+0x65/0x67 [<ffffffff810ab27f>] printk+0x3e/0x46 [..] [<ffffffff8145fb60>] entry_SYSCALL_64_fastpath+0x13/0x94 ---[ end trace ]--- Signed-off-by: Sergey Senozhatsky <[email protected]> --- kernel/printk/alt_printk.c | 9 +++++++++ kernel/printk/internal.h | 1 + 2 files changed, 10 insertions(+) diff --git a/kernel/printk/alt_printk.c b/kernel/printk/alt_printk.c index db0bfc8..0010089 100644 --- a/kernel/printk/alt_printk.c +++ b/kernel/printk/alt_printk.c @@ -150,6 +150,13 @@ static void __alt_printk_flush(struct irq_work *work) more: len = atomic_read(&s->len); + if (this_cpu_read(alt_printk_ctx) & ALT_PRINTK_RECURSION_MASK) { + const char *msg = "BUG: recent printk recursion!\n"; + + this_cpu_and(alt_printk_ctx, ~ALT_PRINTK_RECURSION_MASK); + alt_printk_flush_line(msg, strlen(msg)); + } + /* * This is just a paranoid check that nobody has manipulated * the buffer an unexpected way. If we printed something then @@ -290,6 +297,8 @@ static int vprintk_alt(const char *fmt, va_list args) { struct alt_printk_seq_buf *s = this_cpu_ptr(&alt_print_seq); + /* There is only one way to get here -- a printk recursion. */ + this_cpu_or(alt_printk_ctx, ALT_PRINTK_RECURSION_MASK); return alt_printk_log_store(s, fmt, args); } diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h index 6338f5b..403b6ed 100644 --- a/kernel/printk/internal.h +++ b/kernel/printk/internal.h @@ -37,6 +37,7 @@ static inline int get_nmi_message_lost(void) #define ALT_PRINTK_CONTEXT_MASK 0x07ffffff #define ALT_PRINTK_NMI_CONTEXT_MASK 0x08000000 +#define ALT_PRINTK_RECURSION_MASK 0x10000000 extern raw_spinlock_t logbuf_lock; -- 2.10.0.372.g6fe1b14
