On Sun, 2007-09-02 at 20:27 +0200, Nick Piggin wrote:
> Index: linux-2.6/lib/dyndata.c
> ===================================================================
> --- /dev/null
> +++ linux-2.6/lib/dyndata.c
> @@ -0,0 +1,80 @@
> +#include <linux/dyndata.h>
> +#include <linux/mutex.h>
> +#include <linux/rcupdate.h>
> +#include <linux/seqlock.h>
> +
> +void dyn_data_init(struct dyn_data *dd, void *cur_data)
> +{
> + dd->cur = cur_data;
> + dd->old = NULL;
> + seqcount_init(&dd->resize_seq);
> + mutex_init(&dd->resize_mutex);
> +}
> +
> +/*
> + * Slowpath lookup. There is a resize in progress and we weren't able to
> + * find the item we wanted in dyn_data_lookup: run the full race-free
> + * lookup protocol.
> + */
> +static noinline void *dyn_data_lookup_slow(struct dyn_data *dd, dd_lookup_fn fn, void *arg)
> +{
> + void *ret;
> + void *cur, *old;
> + unsigned seq;
> +
> + cur = dd->cur;
> + old = dd->old;
> +
> + do {
> + seq = read_seqcount_begin(&dd->resize_seq);
> + ret = fn(cur, arg);
> + if (ret)
> + return ret;
> + if (!old)
> + return NULL;
> +
> + ret = fn(old, arg);
> + if (ret)
> + return ret;
> + } while (read_seqcount_retry(&dd->resize_seq, seq));
With the seqlock protecting the whole xfer a lookup for a non-existing
item might spin here for a very long time.
Maybe we should create this sleeping seqlock we talked and provide both
a sleeping and non sleeping version of this lookup function.
> + return NULL;
> +}
> +
> +void *dyn_data_lookup(struct dyn_data *dd, dd_lookup_fn fn, void *arg)
> +{
> + void *ret;
> +
> + rcu_read_lock();
> + if (likely(!dd->old))
> + ret = fn(dd->cur, arg);
> + else
> + ret = dyn_data_lookup_slow(dd, fn, arg);
> + rcu_read_unlock();
> +
> + return ret;
> +}
> +
> +void *dyn_data_replace(struct dyn_data *dd, dd_transfer_fn fn, void *new)
> +{
> + int xfer_done;
> + void *old;
> +
> + BUG_ON(!mutex_is_locked(&dd->resize_mutex));
So its up to the caller to take resize_mutex, but there is no mention of
this requirement. Why cannot it be taken inside this function?
> + old = dd->cur;
> + BUG_ON(dd->old);
> + dd->old = old;
> + synchronize_rcu();
> + rcu_assign_pointer(dd->cur, new);
> + synchronize_rcu();
> + do {
> + write_seqcount_begin(&dd->resize_seq);
> + xfer_done = fn(old, new);
> + write_seqcount_end(&dd->resize_seq);
> + } while (!xfer_done);
> + dd->old = NULL;
> + synchronize_rcu();
> +
> +
> + return old;
> +}
> Index: linux-2.6/include/linux/dyndata.h
> ===================================================================
> --- /dev/null
> +++ linux-2.6/include/linux/dyndata.h
> @@ -0,0 +1,94 @@
> +#ifndef __DYNDATA_H__
> +#define __DYNDATA_H__
> +
<snip>
> +
> +#include <linux/mutex.h>
> +#include <linux/seqlock.h>
> +
> +struct dyn_data {
> + void *cur;
> + void *old;
> + seqcount_t resize_seq;
> + struct mutex resize_mutex;
> +};
> +
> +typedef void *(dd_lookup_fn)(void *dstruct, void *arg);
> +typedef int (dd_transfer_fn)(void *old_ds, void *new_ds);
> +
> +void dyn_data_init(struct dyn_data *dd, void *cur_data);
> +void *dyn_data_lookup(struct dyn_data *dd, dd_lookup_fn fn, void *arg);
> +void *dyn_data_replace(struct dyn_data *dd, dd_transfer_fn fn, void *new);
> +
> +static inline void *dyn_data_current_dstruct(struct dyn_data *dd)
> +{
> + return dd->cur;
> +}
> +
> +#endif
-
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