The patch titled
Subject: kdb: call vkdb_printf() from vprintk_default() only when wanted
has been removed from the -mm tree. Its filename was
kdb-call-vkdb_printf-from-vprintk_default-only-when-wanted.patch
This patch was dropped because it was merged into mainline or a subsystem tree
------------------------------------------------------
From: Petr Mladek <pmladek@xxxxxxxx>
Subject: kdb: call vkdb_printf() from vprintk_default() only when wanted
kdb_trap_printk allows to pass normal printk() messages to kdb via
vkdb_printk(). For example, it is used to get backtrace using the classic
show_stack(), see kdb_show_stack().
vkdb_printf() tries to avoid a potential infinite loop by disabling the
trap. But this approach is racy, for example:
CPU1 CPU2
vkdb_printf()
// assume that kdb_trap_printk == 0
saved_trap_printk = kdb_trap_printk;
kdb_trap_printk = 0;
kdb_show_stack()
kdb_trap_printk++;
Problem1: Now, a nested printk() on CPU0 calls vkdb_printf()
even when it should have been disabled. It will not
cause a deadlock but...
// using the outdated saved value: 0
kdb_trap_printk = saved_trap_printk;
kdb_trap_printk--;
Problem2: Now, kdb_trap_printk == -1 and will stay like this.
It means that all messages will get passed to kdb from
now on.
This patch removes the racy saved_trap_printk handling. Instead, the
recursion is prevented by a check for the locked CPU.
The solution is still kind of racy. A non-related printk(), from another
process, might get trapped by vkdb_printf(). And the wanted printk()
might not get trapped because kdb_printf_cpu is assigned. But this
problem existed even with the original code.
A proper solution would be to get_cpu() before setting kdb_trap_printk and
trap messages only from this CPU. I am not sure if it is worth the
effort, though.
In fact, the race is very theoretical. When kdb is running any of the
commands that use kdb_trap_printk there is a single active CPU and the
other CPUs should be in a holding pen inside kgdb_cpu_enter().
The only time this is violated is when there is a timeout waiting for the
other CPUs to report to the holding pen.
Finally, note that the situation is a bit schizophrenic. vkdb_printf()
explicitly allows recursion but only from KDB code that calls kdb_printf()
directly. On the other hand, the generic printk() recursion is not
allowed because it might cause an infinite loop. This is why we could not
hide the decision inside vkdb_printf() easily.
Link: http://lkml.kernel.org/r/1480412276-16690-4-git-send-email-pmladek@xxxxxxxx
Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
Cc: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
Cc: Jason Wessel <jason.wessel@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/kdb.h | 1 +
kernel/debug/kdb/kdb_io.c | 9 ++-------
kernel/printk/printk.c | 3 ++-
3 files changed, 5 insertions(+), 8 deletions(-)
diff -puN include/linux/kdb.h~kdb-call-vkdb_printf-from-vprintk_default-only-when-wanted include/linux/kdb.h
--- a/include/linux/kdb.h~kdb-call-vkdb_printf-from-vprintk_default-only-when-wanted
+++ a/include/linux/kdb.h
@@ -161,6 +161,7 @@ enum kdb_msgsrc {
};
extern int kdb_trap_printk;
+extern int kdb_printf_cpu;
extern __printf(2, 0) int vkdb_printf(enum kdb_msgsrc src, const char *fmt,
va_list args);
extern __printf(1, 2) int kdb_printf(const char *, ...);
diff -puN kernel/debug/kdb/kdb_io.c~kdb-call-vkdb_printf-from-vprintk_default-only-when-wanted kernel/debug/kdb/kdb_io.c
--- a/kernel/debug/kdb/kdb_io.c~kdb-call-vkdb_printf-from-vprintk_default-only-when-wanted
+++ a/kernel/debug/kdb/kdb_io.c
@@ -30,6 +30,7 @@
char kdb_prompt_str[CMD_BUFLEN];
int kdb_trap_printk;
+int kdb_printf_cpu = -1;
static int kgdb_transition_check(char *buffer)
{
@@ -554,24 +555,19 @@ int vkdb_printf(enum kdb_msgsrc src, con
int linecount;
int colcount;
int logging, saved_loglevel = 0;
- int saved_trap_printk;
int retlen = 0;
int fnd, len;
int this_cpu, old_cpu;
- static int kdb_printf_cpu = -1;
char *cp, *cp2, *cphold = NULL, replaced_byte = ' ';
char *moreprompt = "more> ";
struct console *c = console_drivers;
unsigned long uninitialized_var(flags);
- local_irq_save(flags);
- saved_trap_printk = kdb_trap_printk;
- kdb_trap_printk = 0;
-
/* Serialize kdb_printf if multiple cpus try to write at once.
* But if any cpu goes recursive in kdb, just print the output,
* even if it is interleaved with any other text.
*/
+ local_irq_save(flags);
this_cpu = smp_processor_id();
for (;;) {
old_cpu = cmpxchg(&kdb_printf_cpu, -1, this_cpu);
@@ -849,7 +845,6 @@ kdb_print_out:
console_loglevel = saved_loglevel;
/* kdb_printf_cpu locked the code above. */
smp_store_release(&kdb_printf_cpu, old_cpu);
- kdb_trap_printk = saved_trap_printk;
local_irq_restore(flags);
return retlen;
}
diff -puN kernel/printk/printk.c~kdb-call-vkdb_printf-from-vprintk_default-only-when-wanted kernel/printk/printk.c
--- a/kernel/printk/printk.c~kdb-call-vkdb_printf-from-vprintk_default-only-when-wanted
+++ a/kernel/printk/printk.c
@@ -1926,7 +1926,8 @@ int vprintk_default(const char *fmt, va_
int r;
#ifdef CONFIG_KGDB_KDB
- if (unlikely(kdb_trap_printk)) {
+ /* Allow to pass printk() to kdb but avoid a recursion. */
+ if (unlikely(kdb_trap_printk && kdb_printf_cpu < 0)) {
r = vkdb_printf(KDB_MSGSRC_PRINTK, fmt, args);
return r;
}
_
Patches currently in -mm which might be from pmladek@xxxxxxxx are
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