Messages in this thread Patch in this message | | | Subject | [PATCH] CFQ: clarify cleverness regarding cic->key | From | Paul Bolle <> | Date | Wed, 03 Aug 2011 20:54:50 +0200 |
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Add (and edit) a few comments to make the cleverness regarding cic->key more obvious.
Signed-off-by: Paul Bolle <pebolle@tiscali.nl> --- 0) Not even compile tested.
1) It seems I ran into another CFQ corner case again. This forces me to study this code once more. I found the clever dual use of cic->key rather non-obvious. But perhaps this is a common idiom. Anyhow, are these comments too verbose?
block/cfq-iosched.c | 11 +++++++++-- include/linux/iocontext.h | 2 ++ 2 files changed, 11 insertions(+), 2 deletions(-)
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c index 1f96ad6..c1618dd 100644 --- a/block/cfq-iosched.c +++ b/block/cfq-iosched.c @@ -480,6 +480,10 @@ static inline void cic_set_cfqq(struct cfq_io_context *cic, #define CIC_DEAD_KEY 1ul #define CIC_DEAD_INDEX_SHIFT 1 +/* + * We mangle cic_index so it will always be an odd number, and then cast it to + * a void pointer, so it can never be a valid pointer to a struct cfq_data. + */ static inline void *cfqd_dead_key(struct cfq_data *cfqd) { return (void *)(cfqd->cic_index << CIC_DEAD_INDEX_SHIFT | CIC_DEAD_KEY); @@ -489,6 +493,8 @@ static inline struct cfq_data *cic_to_cfqd(struct cfq_io_context *cic) { struct cfq_data *cfqd = cic->key; + /* if this is an odd number it can't be valid a pointer to a + * struct cfq_data, so this must be dead cic */ if (unlikely((unsigned long) cfqd & CIC_DEAD_KEY)) return NULL; @@ -2708,6 +2714,7 @@ static void cic_free_func(struct io_context *ioc, struct cfq_io_context *cic) BUG_ON(!(dead_key & CIC_DEAD_KEY)); spin_lock_irqsave(&ioc->lock, flags); + /* we must demangle dead_key into a valid radix tree index again */ radix_tree_delete(&ioc->radix_root, dead_key >> CIC_DEAD_INDEX_SHIFT); hlist_del_rcu(&cic->cic_list); spin_unlock_irqrestore(&ioc->lock, flags); @@ -3141,8 +3148,8 @@ cfq_cic_lookup(struct cfq_data *cfqd, struct io_context *ioc) } /* - * Add cic into ioc, using cfqd as the search key. This enables us to lookup - * the process specific cfq io context when entered from the block layer. + * Add cic into ioc, using cic_index as the search key. This enables us to + * lookup the process specific cfq io context when entered from the block layer. * Also adds the cic to a per-cfqd list, used when this queue is removed. */ static int cfq_cic_link(struct cfq_data *cfqd, struct io_context *ioc, diff --git a/include/linux/iocontext.h b/include/linux/iocontext.h index 5037a0a..a7f2c83 100644 --- a/include/linux/iocontext.h +++ b/include/linux/iocontext.h @@ -14,6 +14,8 @@ struct cfq_ttime { }; struct cfq_io_context { + /* either a valid pointer to a struct cfq_data or a (mangled) index + * to io_context->radix_root */ void *key; struct cfq_queue *cfqq[2]; -- 1.7.4.4
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