[PATCH v3] watchdog: s3c2410: Fix potential deadlock on &wdt->lock

[Chengfeng Ye]

As &wdt->lock is acquired by hard irq s3c2410wdt_irq(),
other acquisition of the same lock under process context should
disable irq, otherwise deadlock could happen if the
irq preempt the execution while the lock is held in process context
on the same CPU.

[Deadlock Scenario]
s3c2410wdt_suspend()
    -> s3c2410wdt_stop()
    -> spin_lock(&wdt->lock)
        <irq iterrupt>
        -> s3c2410wdt_irq()
        -> s3c2410wdt_keepalive()
        -> spin_lock(&wdt->lock) (deadlock here)

[Deadlock Scenario]
s3c2410wdt_probe()
    -> s3c2410wdt_start()
    -> spin_lock(&wdt->lock)
        <irq iterrupt>
        -> s3c2410wdt_irq()
        -> s3c2410wdt_keepalive()
        -> spin_lock(&wdt->lock) (deadlock here)

[Deadlock Scenario]
s3c2410wdt_keepalive()
    -> spin_lock(&wdt->lock)
        <irq iterrupt>
        -> s3c2410wdt_irq()
        -> s3c2410wdt_keepalive()
        -> spin_lock(&wdt->lock) (deadlock here)

This flaw was found by an experimental static analysis tool I am
developing for irq-related deadlock, which reported the above
warning when analyzing the linux kernel 6.4-rc7 release.

The tentative patch fix the potential deadlock by spin_lock_irqsave()
under process context.

Signed-off-by: Chengfeng Ye <dg573847474 at gmail.com>
---
 drivers/watchdog/s3c2410_wdt.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/drivers/watchdog/s3c2410_wdt.c b/drivers/watchdog/s3c2410_wdt.c
index 95416a9bdd4b..e1dc71ece01e 100644
--- a/drivers/watchdog/s3c2410_wdt.c
+++ b/drivers/watchdog/s3c2410_wdt.c
@@ -379,10 +379,11 @@ static int s3c2410wdt_enable(struct s3c2410_wdt *wdt, bool en)
 static int s3c2410wdt_keepalive(struct watchdog_device *wdd)
 {
 	struct s3c2410_wdt *wdt = watchdog_get_drvdata(wdd);
+	unsigned long flags;
 
-	spin_lock(&wdt->lock);
+	spin_lock_irqsave(&wdt->lock, flags);
 	writel(wdt->count, wdt->reg_base + S3C2410_WTCNT);
-	spin_unlock(&wdt->lock);
+	spin_unlock_irqrestore(&wdt->lock, flags);
 
 	return 0;
 }
@@ -399,10 +400,11 @@ static void __s3c2410wdt_stop(struct s3c2410_wdt *wdt)
 static int s3c2410wdt_stop(struct watchdog_device *wdd)
 {
 	struct s3c2410_wdt *wdt = watchdog_get_drvdata(wdd);
+	unsigned long flags;
 
-	spin_lock(&wdt->lock);
+	spin_lock_irqsave(&wdt->lock, flags);
 	__s3c2410wdt_stop(wdt);
-	spin_unlock(&wdt->lock);
+	spin_unlock_irqrestore(&wdt->lock, flags);
 
 	return 0;
 }
@@ -411,8 +413,9 @@ static int s3c2410wdt_start(struct watchdog_device *wdd)
 {
 	unsigned long wtcon;
 	struct s3c2410_wdt *wdt = watchdog_get_drvdata(wdd);
+	unsigned long flags;
 
-	spin_lock(&wdt->lock);
+	spin_lock_irqsave(&wdt->lock, flags);
 
 	__s3c2410wdt_stop(wdt);
 
@@ -433,7 +436,7 @@ static int s3c2410wdt_start(struct watchdog_device *wdd)
 	writel(wdt->count, wdt->reg_base + S3C2410_WTDAT);
 	writel(wdt->count, wdt->reg_base + S3C2410_WTCNT);
 	writel(wtcon, wdt->reg_base + S3C2410_WTCON);
-	spin_unlock(&wdt->lock);
+	spin_unlock_irqrestore(&wdt->lock, flags);
 
 	return 0;
 }
-- 
2.17.1