kdb: Make kdb_printf() console handling more robust

While rounding up CPUs via NMIs, its possible that a rounded up CPU
maybe holding a console port lock leading to kgdb master CPU stuck in
a deadlock during invocation of console write operations. A similar
deadlock could also be possible while using synchronous breakpoints.

So in order to avoid such a deadlock, set oops_in_progress to encourage
the console drivers to disregard their internal spin locks: in the
current calling context the risk of deadlock is a bigger problem than
risks due to re-entering the console driver. We operate directly on
oops_in_progress rather than using bust_spinlocks() because the calls
bust_spinlocks() makes on exit are not appropriate for this calling
context.

Suggested-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/1591264879-25920-4-git-send-email-sumit.garg@linaro.org
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>

diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
index 58b7d25..0e4f2ed 100644 (file)
--- a/kernel/debug/kdb/kdb_io.c
+++ b/kernel/debug/kdb/kdb_io.c
@@ -562,7 +562,18 @@ static void kdb_msg_write(const char *msg, int msg_len)
        for_each_console(c) {
                if (!(c->flags & CON_ENABLED))
                        continue;
+               /*
+                * Set oops_in_progress to encourage the console drivers to
+                * disregard their internal spin locks: in the current calling
+                * context the risk of deadlock is a bigger problem than risks
+                * due to re-entering the console driver. We operate directly on
+                * oops_in_progress rather than using bust_spinlocks() because
+                * the calls bust_spinlocks() makes on exit are not appropriate
+                * for this calling context.
+                */
+               ++oops_in_progress;
                c->write(c, msg, msg_len);
+               --oops_in_progress;
                touch_nmi_watchdog();
        }
 }