* Ingo Molnar <[EMAIL PROTECTED]> wrote:
> > > - t = printk_clock();
> > > + t = cpu_clock(printk_cpu);
> > > nanosec_rem = do_div(t, 1000000000);
> > > tlen = sprintf(tbuf,
> > > "<%c>[%5lu.%06lu] ",
> >
> > A bit risky - it's quite an expansion of code which no longer can call
> > printk.
> >
> > You might want to take that WARN_ON out of __update_rq_clock() ;)
>
> hm, dont we already detect printk recursions and turn them into a
> silent return instead of a hang/crash?
ugh, we dont. So i guess the (tested) patch below is highly needed. (If
such incidents become frequent then we could save the stackdump of the
recursion via save_stack_trace() too - but i wanted to keep the initial
code simple.)
Ingo
---------------->
Subject: printk: make printk more robust by not allowing recursion
From: Ingo Molnar <[EMAIL PROTECTED]>
make printk more robust by allowing recursion only if there's a crash
going on. Also add recursion detection.
I've tested it with an artificially injected printk recursion - instead
of a lockup or spontaneous reboot or other crash, the output was a well
controlled:
[ 41.057335] SysRq : <2>BUG: recent printk recursion!
[ 41.057335] loglevel0-8 reBoot Crashdump show-all-locks(D) tErm Full kIll
saK showMem Nice powerOff showPc show-all-timers(Q) unRaw Sync showTasks
Unmount shoW-blocked-tasks
also do all this printk logic with irqs disabled.
Signed-off-by: Ingo Molnar <[EMAIL PROTECTED]>
---
kernel/printk.c | 52 ++++++++++++++++++++++++++++++++++++++++++----------
1 file changed, 42 insertions(+), 10 deletions(-)
Index: linux/kernel/printk.c
===================================================================
--- linux.orig/kernel/printk.c
+++ linux/kernel/printk.c
@@ -623,30 +623,57 @@ asmlinkage int printk(const char *fmt, .
/* cpu currently holding logbuf_lock */
static volatile unsigned int printk_cpu = UINT_MAX;
+const char printk_recursion_bug_msg [] =
+ KERN_CRIT "BUG: recent printk recursion!\n";
+static int printk_recursion_bug;
+
asmlinkage int vprintk(const char *fmt, va_list args)
{
+ static int log_level_unknown = 1;
+ static char printk_buf[1024];
+
unsigned long flags;
- int printed_len;
+ int printed_len = 0;
+ int this_cpu;
char *p;
- static char printk_buf[1024];
- static int log_level_unknown = 1;
boot_delay_msec();
preempt_disable();
- if (unlikely(oops_in_progress) && printk_cpu == smp_processor_id())
- /* If a crash is occurring during printk() on this CPU,
- * make sure we can't deadlock */
- zap_locks();
-
/* This stops the holder of console_sem just where we want him */
raw_local_irq_save(flags);
+ this_cpu = smp_processor_id();
+
+ /*
+ * Ouch, printk recursed into itself!
+ */
+ if (unlikely(printk_cpu == this_cpu)) {
+ /*
+ * If a crash is occurring during printk() on this CPU,
+ * then try to get the crash message out but make sure
+ * we can't deadlock. Otherwise just return to avoid the
+ * recursion and return - but flag the recursion so that
+ * it can be printed at the next appropriate moment:
+ */
+ if (!oops_in_progress) {
+ printk_recursion_bug = 1;
+ goto out_restore_irqs;
+ }
+ zap_locks();
+ }
+
lockdep_off();
spin_lock(&logbuf_lock);
- printk_cpu = smp_processor_id();
+ printk_cpu = this_cpu;
+ if (printk_recursion_bug) {
+ printk_recursion_bug = 0;
+ strcpy(printk_buf, printk_recursion_bug_msg);
+ printed_len = sizeof(printk_recursion_bug_msg);
+ }
/* Emit the output into the temporary buffer */
- printed_len = vscnprintf(printk_buf, sizeof(printk_buf), fmt, args);
+ printed_len += vscnprintf(printk_buf + printed_len,
+ sizeof(printk_buf), fmt, args);
/*
* Copy the output into log_buf. If the caller didn't provide
@@ -675,6 +702,10 @@ asmlinkage int vprintk(const char *fmt,
loglev_char = default_message_loglevel
+ '0';
}
+ if (panic_timeout) {
+ panic_timeout = 0;
+ printk("recurse!\n");
+ }
t = cpu_clock(printk_cpu);
nanosec_rem = do_div(t, 1000000000);
tlen = sprintf(tbuf,
@@ -739,6 +770,7 @@ asmlinkage int vprintk(const char *fmt,
printk_cpu = UINT_MAX;
spin_unlock(&logbuf_lock);
lockdep_on();
+out_restore_irqs:
raw_local_irq_restore(flags);
}
--
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