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 <[email protected]> --- 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
