linux-kernel - Re: [linux-next][regression] [PATCH] percpu: add preemption checks to __this

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Message-ID: <alpine.DEB.2.10.1403180922050.17965@nuc>
Date:	Tue, 18 Mar 2014 09:37:58 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Grygorii Strashko <grygorii.strashko@...com>
cc:	linux-arm <linux-arm-kernel@...ts.infradead.org>,
	Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [linux-next][regression] [PATCH] percpu: add preemption checks
 to __this_cpu ops

On Tue, 18 Mar 2014, Grygorii Strashko wrote:

> diff --git a/lib/smp_processor_id.c b/lib/smp_processor_id.c
> index a270dce..73a2004 100644
> --- a/lib/smp_processor_id.c
> +++ b/lib/smp_processor_id.c
> @@ -58,9 +58,6 @@ EXPORT_SYMBOL(debug_smp_processor_id);
>
>  notrace void __this_cpu_preempt_check(const char *op)
>  {
> -       char text[40];
> -
> -       snprintf(text, sizeof(text), "__this_cpu_%s()", op);
> -       check_preemption_disabled(text);
> +       check_preemption_disabled(op);
>  }
>  EXPORT_SYMBOL(__this_cpu_preempt_check);

So it looks like there is an early this cpu operation in a context that
cannot handle snprintf. But the checks in check_preemption_disabled()
avoid processing there so that works.

We could fix this by moving the string concatenation operation into
the check function.

Index: linux/lib/smp_processor_id.c
===================================================================
--- linux.orig/lib/smp_processor_id.c	2014-03-18 09:36:31.330450525 -0500
+++ linux/lib/smp_processor_id.c	2014-03-18 09:36:37.822315534 -0500
@@ -7,7 +7,8 @@
 #include <linux/kallsyms.h>
 #include <linux/sched.h>

-notrace static unsigned int check_preemption_disabled(char *what)
+notrace static unsigned int check_preemption_disabled(const char *what1,
+							const char *what2)
 {
 	int this_cpu = raw_smp_processor_id();

@@ -38,8 +39,8 @@
 	if (!printk_ratelimit())
 		goto out_enable;

-	printk(KERN_ERR "BUG: using %s in preemptible [%08x] code: %s/%d\n",
-		what, preempt_count() - 1, current->comm, current->pid);
+	printk(KERN_ERR "BUG: using %s%s() in preemptible [%08x] code: %s/%d\n",
+		what1, what2, preempt_count() - 1, current->comm, current->pid);

 	print_symbol("caller is %s\n", (long)__builtin_return_address(0));
 	dump_stack();
@@ -52,15 +53,12 @@

 notrace unsigned int debug_smp_processor_id(void)
 {
-	return check_preemption_disabled("smp_processor_id()");
+	return check_preemption_disabled("smp_processor_id","");
 }
 EXPORT_SYMBOL(debug_smp_processor_id);

 notrace void __this_cpu_preempt_check(const char *op)
 {
-	char text[40];
-
-	snprintf(text, sizeof(text), "__this_cpu_%s()", op);
-	check_preemption_disabled(text);
+	check_preemption_disabled("__this_cpu_", op);
 }
 EXPORT_SYMBOL(__this_cpu_preempt_check);
--
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