[lustre-devel] [PATCH 1/6] staging: lustre: make key_set_version an atomic_t

[James Simmons]

> As a first step to simplifying the locking in lu_object.c,
> change key_set_version to an atomic_t.  This will mean
> we don't need to hold a spinlock when incrementing.
> 
> It is clear that keys_fill() (and so lu_context_refill())
> cannot race with itself as it holds no locks for the main
> part of the work.  So updates to lc_version, and testing of
> that value cannot need locking either.
> So remove the locking from keys_fill() and lu_context_refill()
> as there is no longer anything to protect.  The locking around
> lu_keys_initing_cnt is preserved for now.
> 
> Also, don't increment when deregistering or quiescing a key.
> key_set_version is *only* use to avoid filling new key values
> if there have been no changes to the set of key.
> Deregistering a key does not mean that we need to try filling
> any new value, so the increment is pointless.
> 
> Finally, remove the refill loop in keys_fill().  If a key
> is registered or revived while keys_fill() is running it must be safe
> to ignore it just as we would if it was registered immediately
> after keys_fill() ran.  The important thing is that the
> keys_set_version stored in ctx->lc_version must be sampled
> *before* those unseen keys were added.
> So sample keys_set_version early.
> 
> Signed-off-by: NeilBrown <neilb at suse.com>

Why did you create this patch when the next patch replaces this change
with a semaphore? Couldn't we just drop this step and go with the
second patch as the first in the series?

> ---
>  drivers/staging/lustre/lustre/obdclass/lu_object.c |   26 ++++----------------
>  1 file changed, 5 insertions(+), 21 deletions(-)
> 
> diff --git a/drivers/staging/lustre/lustre/obdclass/lu_object.c b/drivers/staging/lustre/lustre/obdclass/lu_object.c
> index f14e3509f059..b38adf75e8e4 100644
> --- a/drivers/staging/lustre/lustre/obdclass/lu_object.c
> +++ b/drivers/staging/lustre/lustre/obdclass/lu_object.c
> @@ -1329,7 +1329,7 @@ static atomic_t lu_key_initing_cnt = ATOMIC_INIT(0);
>   * lu_context_refill(). No locking is provided, as initialization and shutdown
>   * are supposed to be externally serialized.
>   */
> -static unsigned int key_set_version;
> +static atomic_t key_set_version = ATOMIC_INIT(0);
>  
>  /**
>   * Register new key.
> @@ -1352,7 +1352,7 @@ int lu_context_key_register(struct lu_context_key *key)
>  			lu_keys[i] = key;
>  			lu_ref_init(&key->lct_reference);
>  			result = 0;
> -			++key_set_version;
> +			atomic_inc(&key_set_version);
>  			break;
>  		}
>  	}
> @@ -1390,7 +1390,6 @@ void lu_context_key_degister(struct lu_context_key *key)
>  	lu_context_key_quiesce(key);
>  
>  	write_lock(&lu_keys_guard);
> -	++key_set_version;
>  	key_fini(&lu_shrink_env.le_ctx, key->lct_index);
>  
>  	/**
> @@ -1550,7 +1549,6 @@ void lu_context_key_quiesce(struct lu_context_key *key)
>  		list_for_each_entry(ctx, &lu_context_remembered, lc_remember)
>  			key_fini(ctx, key->lct_index);
>  
> -		++key_set_version;
>  		write_unlock(&lu_keys_guard);
>  	}
>  }
> @@ -1559,7 +1557,7 @@ void lu_context_key_revive(struct lu_context_key *key)
>  {
>  	write_lock(&lu_keys_guard);
>  	key->lct_tags &= ~LCT_QUIESCENT;
> -	++key_set_version;
> +	atomic_inc(&key_set_version);
>  	write_unlock(&lu_keys_guard);
>  }
>  
> @@ -1579,7 +1577,6 @@ static void keys_fini(struct lu_context *ctx)
>  
>  static int keys_fill(struct lu_context *ctx)
>  {
> -	unsigned int pre_version;
>  	unsigned int i;
>  
>  	/*
> @@ -1593,10 +1590,9 @@ static int keys_fill(struct lu_context *ctx)
>  	 */
>  	read_lock(&lu_keys_guard);
>  	atomic_inc(&lu_key_initing_cnt);
> -	pre_version = key_set_version;
>  	read_unlock(&lu_keys_guard);
> +	ctx->lc_version = atomic_read(&key_set_version);
>  
> -refill:
>  	LINVRNT(ctx->lc_value);
>  	for (i = 0; i < ARRAY_SIZE(lu_keys); ++i) {
>  		struct lu_context_key *key;
> @@ -1639,15 +1635,7 @@ static int keys_fill(struct lu_context *ctx)
>  		}
>  	}
>  
> -	read_lock(&lu_keys_guard);
> -	if (pre_version != key_set_version) {
> -		pre_version = key_set_version;
> -		read_unlock(&lu_keys_guard);
> -		goto refill;
> -	}
> -	ctx->lc_version = key_set_version;
>  	atomic_dec(&lu_key_initing_cnt);
> -	read_unlock(&lu_keys_guard);
>  	return 0;
>  }
>  
> @@ -1756,13 +1744,9 @@ EXPORT_SYMBOL(lu_context_exit);
>   */
>  int lu_context_refill(struct lu_context *ctx)
>  {
> -	read_lock(&lu_keys_guard);
> -	if (likely(ctx->lc_version == key_set_version)) {
> -		read_unlock(&lu_keys_guard);
> +	if (likely(ctx->lc_version == atomic_read(&key_set_version)))
>  		return 0;
> -	}
>  
> -	read_unlock(&lu_keys_guard);
>  	return keys_fill(ctx);
>  }
>  
> 
> 
>