[linux-audio-dev] SMP low latency patch: tested - race condition

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Subject: [linux-audio-dev] SMP low latency patch: tested - race condition
From: Paul Barton-Davis (pbd_AT_Op.Net)
Date: ti marras 30 1999 - 15:47:06 EST

Benno -

i finally got to testing out Ingo's suggestions for the SMP low
latency fix. alas, it appears to introduce a race condition. I tried 2
bootups. The first one hung during the microcode download to my
Tropez+; the second one hung during a probe for an ethernet card
(which i don't have installed). 2.2.10 with the LL patch never does
this.

I don't have Ingo's mailing address, so perhaps you could let him
know. Enclosed below is my /usr/src/linux/include/asm/smplock.h. If I
had more time right now, I would look into it myself, but I don't.

--p

/*
 * <asm/smplock.h>
 *
 * i386 SMP lock implementation
 */
#include <linux/interrupt.h>
#include <linux/condsched.h>
#include <asm/spinlock.h>

extern spinlock_t kernel_flag;

/*
 * Release global kernel lock and global interrupt lock
 */
#define release_kernel_lock(task, cpu) \
do { \
        if (task->lock_depth >= 0) \
                spin_unlock(&kernel_flag); \
        release_irqlock(cpu); \
        __sti(); \
} while (0)

/*
 * Re-acquire the kernel lock
 */

#ifdef NOT_LOW_LATENCY

#define reacquire_kernel_lock(task) \
do { \
        if (task->lock_depth >= 0) \
                spin_lock(&kernel_flag); \
} while (0)

#else

#define reacquire_kernel_lock(task) \
do { \
        if (task->lock_depth >= 0) \
                while (!spin_trylock(&kernel_flag)) \
                        if (current->need_resched) \
                                schedule(); \
} while (0)

#endif /* NOT_LOW_LATENCY */

/*
 * Getting the big kernel lock.
 *
 * This cannot happen asynchronously,
 * so we only need to worry about other
 * CPU's.
 */
extern __inline__ void lock_kernel(void)
{

#ifdef NOT_LOW_LATENCY

        __asm__ __volatile__(
                "incl %1\n\t"
                "jne 9f"
                spin_lock_string
                "\n9:"
                :"=m" (__dummy_lock(&kernel_flag)),
                 "=m" (current->lock_depth));

#else

        if (!++current->lock_depth) {
                reacquire_kernel_lock(current);
        }

#endif /* NOT_LOW_LATENCY */
}

extern __inline__ void unlock_kernel(void)
{
        __asm__ __volatile__(
                "decl %1\n\t"
                "jns 9f\n\t"
                spin_unlock_string
                "\n9:"
                :"=m" (__dummy_lock(&kernel_flag)),
                 "=m" (current->lock_depth));
}

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