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    SubjectRe: [PATCH 3/6] C-language equivalents of include/asm-*/bitops.h
    Date
    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.

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