On Fri, Jun 14, 2019 at 02:41:20PM +0200, Christian König wrote:
> The ww_mutex implementation allows for detection deadlocks when multiple
> threads try to acquire the same set of locks in different order.
>
> The problem is that handling those deadlocks was the burden of the user of
> the ww_mutex implementation and at least some users didn't got that right
> on the first try.
>
> I'm not a big fan of macros, but it still better then implementing the same
> logic at least halve a dozen times. So this patch adds two macros to lock
> and unlock multiple ww_mutex instances with the necessary deadlock handling.
>
> Signed-off-by: Christian König <christian.koenig@xxxxxxx>
> ---
> include/linux/ww_mutex.h | 75 ++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 75 insertions(+)
>
> diff --git a/include/linux/ww_mutex.h b/include/linux/ww_mutex.h
> index 3af7c0e03be5..a0d893b5b114 100644
> --- a/include/linux/ww_mutex.h
> +++ b/include/linux/ww_mutex.h
> @@ -369,4 +369,79 @@ static inline bool ww_mutex_is_locked(struct ww_mutex *lock)
> return mutex_is_locked(&lock->base);
> }
>
> +/**
> + * ww_mutex_unlock_for_each - cleanup after error or contention
> + * @loop: for loop code fragment iterating over all the locks
> + * @pos: code fragment returning the currently handled lock
> + * @contended: the last contended ww_mutex or NULL or ERR_PTR
> + *
> + * Helper to make cleanup after error or lock contention easier.
> + * First unlock the last contended lock and then all other locked ones.
> + */
> +#define ww_mutex_unlock_for_each(loop, pos, contended) \
> + if (!IS_ERR(contended)) { \
> + if (contended) \
> + ww_mutex_unlock(contended); \
> + contended = (pos); \
> + loop { \
> + if (contended == (pos)) \
> + break; \
> + ww_mutex_unlock(pos); \
> + } \
> + }
> +
> +/**
> + * ww_mutex_lock_for_each - implement ww_mutex deadlock handling
> + * @loop: for loop code fragment iterating over all the locks
> + * @pos: code fragment returning the currently handled lock
> + * @contended: ww_mutex pointer variable for state handling
> + * @ret: int variable to store the return value of ww_mutex_lock()
> + * @intr: If true ww_mutex_lock_interruptible() is used
> + * @ctx: ww_acquire_ctx pointer for the locking
> + *
> + * This macro implements the necessary logic to lock multiple ww_mutex
> + * instances. Lock contention with backoff and re-locking is handled by the
> + * macro so that the loop body only need to handle other errors and
> + * successfully acquired locks.
> + *
> + * With the @loop and @pos code fragment it is possible to apply this logic
> + * to all kind of containers (array, list, tree, etc...) holding multiple
> + * ww_mutex instances.
> + *
> + * @contended is used to hold the current state we are in. -ENOENT is used to
> + * signal that we are just starting the handling. -EDEADLK means that the
> + * current position is contended and we need to restart the loop after locking
> + * it. NULL means that there is no contention to be handled. Any other value is
> + * the last contended ww_mutex.
> + */
> +#define ww_mutex_lock_for_each(loop, pos, contended, ret, intr, ctx) \
> + for (contended = ERR_PTR(-ENOENT); ({ \
> + __label__ relock, next; \
> + ret = -ENOENT; \
> + if (contended == ERR_PTR(-ENOENT)) \
> + contended = NULL; \
> + else if (contended == ERR_PTR(-EDEADLK)) \
> + contended = (pos); \
> + else \
> + goto next; \
> + loop { \
> + if (contended == (pos)) { \
> + contended = NULL; \
> + continue; \
> + } \
> +relock: \
> + ret = !(intr) ? ww_mutex_lock(pos, ctx) : \
> + ww_mutex_lock_interruptible(pos, ctx); \
> + if (ret == -EDEADLK) { \
> + ww_mutex_unlock_for_each(loop, pos, \
> + contended); \
> + contended = ERR_PTR(-EDEADLK); \
> + goto relock; \
> + } \
> + break; \
> +next: \
> + continue; \
> + } \
> + }), ret != -ENOENT;)
> +
Yea gawds, that's eyebleeding bad, even for macros :/
It also breaks with pretty much all other *for_each*() macros in
existence by not actually including the loop itself but farming that out
to an argument statement (@loop), suggesting these really should not be
called for_each.
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