Akinobu Mita (on Wed, 25 Jan 2006 20:32:06 +0900) wrote:
o generic {,test_and_}{set,clear,change}_bit() (atomic bitops)
...
+static __inline__ void set_bit(int nr, volatile unsigned long *addr)
+{
+ unsigned long mask = BITOP_MASK(nr);
+ unsigned long *p = ((unsigned long *)addr) + BITOP_WORD(nr);
+ unsigned long flags;
+
+ _atomic_spin_lock_irqsave(p, flags);
+ *p |= mask;
+ _atomic_spin_unlock_irqrestore(p, flags);
+}
Be very, very careful about using these generic *_bit() routines if the
architecture supports non-maskable interrupts.
NMI events can occur at any time, including when interrupts have been
disabled by *_irqsave(). So you can get NMI events occurring while a
*_bit fucntion is holding a spin lock. If the NMI handler also wants
to do bit manipulation (and they do) then you can get a deadlock
between the original caller of *_bit() and the NMI handler.
Doing any work that requires spinlocks in an NMI handler is just asking
for deadlock problems. The generic *_bit() routines add a hidden
spinlock behind what was previously a safe operation. I would even say
that any arch that supports any type of NMI event _must_ define its own
bit routines that do not rely on your _atomic_spin_lock_irqsave() and
its hash of spinlocks.
On Wed, Jan 25, 2006 at 10:54:43PM +1100, Keith Owens wrote:
Be very, very careful about using these generic *_bit() routines if the
architecture supports non-maskable interrupts.
NMI events can occur at any time, including when interrupts have been
disabled by *_irqsave(). So you can get NMI events occurring while a
*_bit fucntion is holding a spin lock. If the NMI handler also wants
to do bit manipulation (and they do) then you can get a deadlock
between the original caller of *_bit() and the NMI handler.
Doing any work that requires spinlocks in an NMI handler is just asking
for deadlock problems. The generic *_bit() routines add a hidden
spinlock behind what was previously a safe operation. I would even say
that any arch that supports any type of NMI event _must_ define its own
bit routines that do not rely on your _atomic_spin_lock_irqsave() and
its hash of spinlocks.
At least cris and parisc are using similar *_bit function on SMP.
I will add your advise in comment.
--- ./include/asm-generic/bitops.h.orig 2006-01-26 10:56:00.000000000 +0900
+++ ./include/asm-generic/bitops.h 2006-01-26 11:01:28.000000000 +0900
@@ -50,6 +50,16 @@ extern raw_spinlock_t __atomic_hash[ATOM
* C language equivalents written by Theodore Ts'o, 9/26/92
*/
+/*
+ * NMI events can occur at any time, including when interrupts have been
+ * disabled by *_irqsave(). So you can get NMI events occurring while a
+ * *_bit fucntion is holding a spin lock. If the NMI handler also wants
+ * to do bit manipulation (and they do) then you can get a deadlock
+ * between the original caller of *_bit() and the NMI handler.
+ *
+ * by Keith Owens
+ */
+
static __inline__ void set_bit(int nr, volatile unsigned long *addr)
{
unsigned long mask = BITOP_MASK(nr);