linux-kernel - [PATCH v4 3/4] kdb: Make kdb_printf() console handling more robust

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Message-Id: <1590751607-29676-4-git-send-email-sumit.garg@linaro.org>
Date:   Fri, 29 May 2020 16:56:46 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     daniel.thompson@...aro.org
Cc:     kgdb-bugreport@...ts.sourceforge.net, jason.wessel@...driver.com,
        dianders@...omium.org, pmladek@...e.com,
        sergey.senozhatsky@...il.com, gregkh@...uxfoundation.org,
        jslaby@...e.com, linux-kernel@...r.kernel.org,
        Sumit Garg <sumit.garg@...aro.org>
Subject: [PATCH v4 3/4] 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: Petr Mladek <pmladek@...e.com>
Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
---
 kernel/debug/kdb/kdb_io.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
index fad38eb..9e5a40d 100644
--- a/kernel/debug/kdb/kdb_io.c
+++ b/kernel/debug/kdb/kdb_io.c
@@ -566,7 +566,18 @@ static void kdb_msg_write(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();
 	}
 }
-- 
2.7.4