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From:   Sumit Garg <sumit.garg@linaro.org>
To:     kgdb-bugreport@lists.sourceforge.net
Cc:     jason.wessel@windriver.com, daniel.thompson@linaro.org,
        dianders@chromium.org, pmladek@suse.com,
        sergey.senozhatsky@gmail.com, linux-kernel@vger.kernel.org,
        Sumit Garg <sumit.garg@linaro.org>
Subject: [PATCH] kdb: Make kdb_printf robust to run in NMI context
Date:   Fri, 22 May 2020 15:32:26 +0530
Message-Id: <1590141746-23559-1-git-send-email-sumit.garg@linaro.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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. So in order
to avoid such a deadlock, invoke bust_spinlocks() prior to invocation
of console handlers.

Also, add a check for console port to be enabled prior to invocation of
corresponding handler.

Suggested-by: Petr Mladek <pmladek@suse.com>
Suggested-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
---
 kernel/debug/kdb/kdb_io.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
index 924bc92..e32ece6 100644
--- a/kernel/debug/kdb/kdb_io.c
+++ b/kernel/debug/kdb/kdb_io.c
@@ -699,7 +699,11 @@ int vkdb_printf(enum kdb_msgsrc src, const char *fmt, va_list ap)
 			}
 		}
 		for_each_console(c) {
+			if (!(c->flags & CON_ENABLED))
+				continue;
+			bust_spinlocks(1);
 			c->write(c, cp, retlen - (cp - kdb_buffer));
+			bust_spinlocks(0);
 			touch_nmi_watchdog();
 		}
 	}
@@ -761,7 +765,11 @@ int vkdb_printf(enum kdb_msgsrc src, const char *fmt, va_list ap)
 			}
 		}
 		for_each_console(c) {
+			if (!(c->flags & CON_ENABLED))
+				continue;
+			bust_spinlocks(1);
 			c->write(c, moreprompt, strlen(moreprompt));
+			bust_spinlocks(0);
 			touch_nmi_watchdog();
 		}
 
-- 
2.7.4