Messages in this thread Patch in this message | | | From | Petr Mladek <> | Subject | [PATCH 2/2] kdb: Call vkdb_printf() from vprintk_default() only when wanted | Date | Fri, 21 Oct 2016 14:50:35 +0200 |
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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// using the outdated saved value: 0 kdb_trap_printk = saved_trap_printk;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.
Signed-off-by: Petr Mladek <pmladek@suse.com> --- include/linux/kdb.h | 1 + kernel/debug/kdb/kdb_io.c | 9 ++------- kernel/printk/printk.c | 4 +++- 3 files changed, 6 insertions(+), 8 deletions(-)
diff --git a/include/linux/kdb.h b/include/linux/kdb.h index 410decacff8f..ccd4cf838c66 100644 --- a/include/linux/kdb.h +++ b/include/linux/kdb.h @@ -162,6 +162,7 @@ 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 --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c index 227b59ec7dbe..209507e30bd4 100644 --- a/kernel/debug/kdb/kdb_io.c +++ b/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, const char *fmt, va_list ap) 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(); atomic_inc(&kdb_event); for (;;) { @@ -853,7 +849,6 @@ int vkdb_printf(enum kdb_msgsrc src, const char *fmt, va_list ap) /* Update kdb_event around the locked section. */ smp_mb__before_atomic(); atomic_dec(&kdb_event); - kdb_trap_printk = saved_trap_printk; local_irq_restore(flags); return retlen; } diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index d5e397315473..db73e33811e7 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1941,7 +1941,9 @@ int vprintk_default(const char *fmt, va_list args) 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 != smp_processor_id())) { r = vkdb_printf(KDB_MSGSRC_PRINTK, fmt, args); return r; } -- 1.8.5.6
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