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SubjectRe: [RFC][PATCH 4/7] lockdep: Seperate lock ids for read/write acquires
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On Sun, 2011-04-17 at 11:45 +0200, Peter Zijlstra wrote:
>
> +/*
> + * A lock's class id is used to calculate the chain-key. Since we need to
> + * differentiate between the chains which contain the read acquire of
> + * a lock from the chains having write acquire of the same lock,
> + * we offset the class_idx by MAX_LOCKDEP_KEYS if it is a read acquire.

Don't we only care to do this if we have a recursive read? I thought
simple reads still work fine with the current algorithm?

> + *
> + * Thus the the lock's key during a chain-key calculation can be in the range
> + * 1 to 2 * MAX_LOCKDEP_KEYS - 1.
> + *
> + * LOCKDEP_CHAIN_KEY_BITS holds the number of bits required to
> + * represent this range.
> + */
> +#define LOCKDEP_CHAIN_KEY_BITS (MAX_LOCKDEP_KEYS_BITS + 1)
> struct held_lock {
> /*
> * One-way hash of the dependency chain up to this point. We
> Index: linux-2.6/kernel/lockdep.c
> ===================================================================
> --- linux-2.6.orig/kernel/lockdep.c
> +++ linux-2.6/kernel/lockdep.c
> @@ -303,8 +303,8 @@ static struct list_head chainhash_table[
> * unique.
> */
> #define iterate_chain_key(key1, key2) \
> - (((key1) << MAX_LOCKDEP_KEYS_BITS) ^ \
> - ((key1) >> (64-MAX_LOCKDEP_KEYS_BITS)) ^ \
> + (((key1) << LOCKDEP_CHAIN_KEY_BITS) ^ \
> + ((key1) >> (64 - LOCKDEP_CHAIN_KEY_BITS)) ^ \
> (key2))
>
> void lockdep_off(void)
> @@ -1988,6 +1988,9 @@ static void check_chain_key(struct task_
> if (DEBUG_LOCKS_WARN_ON(id >= MAX_LOCKDEP_KEYS))
> return;
>
> + if (is_read(hlock->rw_state))
> + id += MAX_LOCKDEP_KEYS;

Again, isn't this about recursive reads? Or am I just confused ;)

-- Steve

> +
> if (prev_hlock && (prev_hlock->irq_context !=
> hlock->irq_context))
> chain_key = 0;
> @@ -2815,6 +2818,18 @@ static int __lock_acquire(struct lockdep
> if (DEBUG_LOCKS_WARN_ON(id >= MAX_LOCKDEP_KEYS))
> return 0;
>
> + /*
> + * Factor in the read/write state in the chain key calculation.
> + *
> + * Two chains containing lock dependencies in the same order can
> + * still differ due to their read/write state
> + * eg: lock(A)->Rlock(B) is different from lock(A)->Wlock(B)
> + *
> + * Hence distinguish between such chains.
> + */
> + if (is_read(rw_state))
> + id += MAX_LOCKDEP_KEYS;
> +
> chain_key = curr->curr_chain_key;
> if (!depth) {
> if (DEBUG_LOCKS_WARN_ON(chain_key != 0))
>




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