On 12/12/2017 05:43 PM, David Lechner wrote:
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).
I think I have figured out that since CONFIG_SMP=n and
CONFIG_DEBUG_SPINLOCK=n on my kernel that
#define arch_spin_trylock(lock)({ barrier(); (void)(lock); 1; })
in include/linux/spinlock_up.h is causing the problem.
So, basically, reentrancy of clk_enable() is broken for non-SMP systems,
but I'm not sure I know how to fix it.
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 <da...@lechnology.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;
}
