linux-kernel - [PATCH] clk: fix spin_lock/unlock imbalance on bad clk

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Date:   Tue, 12 Dec 2017 17:43:43 -0600
From:   David Lechner <david@...hnology.com>
To:     linux-clk@...r.kernel.org
Cc:     David Lechner <david@...hnology.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...eaurora.org>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] clk: fix spin_lock/unlock imbalance on bad clk_enable() reentrancy

If clk_enable() is called in reentrant way and spin_trylock_irqsave() is
not working as expected, it is possible to get a negative enable_refcnt
which results in a missed call to spin_unlock_irqrestore().

It works like this:

1. clk_enable() is called.
2. clk_enable_unlock() calls spin_trylock_irqsave() and sets
   enable_refcnt = 1.
3. Another clk_enable() is called before the first has returned
   (reentrant), but somehow spin_trylock_irqsave() is returning true.
   (I'm not sure how/why this is happening yet, but it is happening to me
   with arch/arm/mach-davinci clocks that I am working on).
4. Because spin_trylock_irqsave() returned true, enable_lock has been
   locked twice without being unlocked and enable_refcnt = 1 is called
   instead of enable_refcnt++.
5. After the inner clock is enabled clk_enable_unlock() is called which
   decrements enable_refnct to 0 and calls spin_unlock_irqrestore()
6. The inner clk_enable() function returns.
7. clk_enable_unlock() is called again for the outer clock. enable_refcnt
   is decremented to -1 and spin_unlock_irqrestore() is *not* called.
8. The outer clk_enable() function returns.
9. Unrelated code called later issues a BUG warning about sleeping in an
   atomic context because of the unbalanced calls for the spin lock.

This patch fixes the problem of unbalanced calls by calling
spin_unlock_irqrestore() if enable_refnct <= 0 instead of just checking if
it is == 0.

The BUG warning about sleeping in an atomic context in the unrelated code
is eliminated with this patch, but there are still warnings printed from
clk_enable_unlock() and clk_enable_unlock() because of the reference
counting problems.

Signed-off-by: David Lechner <david@...hnology.com>
---
 drivers/clk/clk.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
index 647d056..bb1b1f9 100644
--- a/drivers/clk/clk.c
+++ b/drivers/clk/clk.c
@@ -162,7 +162,7 @@ static void clk_enable_unlock(unsigned long flags)
 	WARN_ON_ONCE(enable_owner != current);
 	WARN_ON_ONCE(enable_refcnt == 0);

-	if (--enable_refcnt) {
+	if (--enable_refcnt > 0) {
 		__release(enable_lock);
 		return;
 	}
-- 
2.7.4