Re: [patch 08/19] mutex subsystem, core

[Nick Piggin <nickpiggin@yahoo.com.au>]

Ingo Molnar wrote:

> +
> +/*
> + * Spinlock based trylock, we take the spinlock and check whether we
> + * can get the lock:
> + */
> +static inline int __mutex_trylock_nonatomic(atomic_t *lock_count)
> +{
> +	struct mutex *lock = container_of(lock_count, struct mutex, count);
> +	int prev;
> +
> +	spin_lock_mutex(&lock->wait_lock);
> +
> +	prev = atomic_xchg(&lock->count, -1);
> +	if (likely(prev == 1))
> +		debug_mutex_set_owner(lock, current_thread_info() __RET_IP__);
> +	/* Set it back to 0 if there are no waiters: */
> +	if (likely(list_empty(&lock->wait_list)))
> +		atomic_set(&lock->count, 0);
> +
> +	spin_unlock_mutex(&lock->wait_lock);
> +
> +	return prev == 1;
> +}
> +
> +/***
> + * mutex_trylock - try acquire the mutex, without waiting
> + * @lock: the mutex to be acquired
> + *
> + * Try to acquire the mutex atomically. Returns 1 if the mutex
> + * has been acquired successfully, and 0 on contention.
> + *
> + * NOTE: this function follows the spin_trylock() convention, so
> + * it is negated to the down_trylock() return values! Be careful
> + * about this when converting semaphore users to mutexes.
> + *
> + * This function must not be used in interrupt context. The
> + * mutex must be released by the same task that acquired it.
> + */
> +int fastcall mutex_trylock(struct mutex *lock)
> +{
> +	return __mutex_fastpath_trylock(&lock->count,
> +					__mutex_trylock_nonatomic);
> +}
> +

[snip]

> +
> +#define spin_lock_mutex(lock)			spin_lock(lock)
> +#define spin_unlock_mutex(lock)			spin_unlock(lock)

Is this an interrupt deadlock, or do you not allow interrupt contexts
to even trylock a mutex?

If the former, I would simply use atomic_cmpxchg unconditionally for
the trylock and not use a spinlock fallback function at all.

Alternately, you may 'trylock' the spinlock, but it just seems like
extra code for not much reason (when you can use atomic_cmpxchg).

--
SUSE Labs, Novell Inc.

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