Re: [PATCH 5/5] btrfs: document extent buffer locking

[Nikolay Borisov <nborisov@suse.com>]

On 17.10.19 г. 22:39 ч., David Sterba wrote:
> Signed-off-by: David Sterba <dsterba@suse.com>
> ---
>  fs/btrfs/locking.c | 110 +++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 96 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c
> index e0e0430577aa..2a0e828b4470 100644
> --- a/fs/btrfs/locking.c
> +++ b/fs/btrfs/locking.c
> @@ -13,6 +13,48 @@
>  #include "extent_io.h"
>  #include "locking.h"
>  
> +/*
> + * Extent buffer locking
> + * ~~~~~~~~~~~~~~~~~~~~~
> + *
> + * The locks use a custom scheme that allows to do more operations than are
> + * available fromt current locking primitives. The building blocks are still
> + * rwlock and wait queues.
> + *
> + * Required semantics:

Thinking out loud here..

> + *
> + * - reader/writer exclusion

RWSEM has that

> + * - writer/writer exclusion

RWSEM has that

> + * - reader/reader sharing

RWSEM has that
> + * - spinning lock semantics

RWSEM has that in the form of optimistic spinning which would be shorter
than what the custom implementation provides.

> + * - blocking lock semantics

RWSEM has that

> + * - try-lock semantics for readers and writers

down_write_trylock, down_read_trylock.

> + * - one level nesting, allowing read lock to be taken by the same thread that
> + *   already has write lock

this might be somewhat problematic but there is downgrade_write which
could be used, I'll have to check.

> + *
> + * The extent buffer locks (also called tree locks) manage access to eb data.
> + * We want concurrency of many readers and safe updates. The underlying locking
> + * is done by read-write spinlock and the blocking part is implemented using
> + * counters and wait queues.
> + *
> + * spinning semantics - the low-level rwlock is held so all other threads that
> + *                      want to take it are spinning on it.
> + *
> + * blocking semantics - the low-level rwlock is not held but the counter
> + *                      denotes how many times the blocking lock was held;
> + *                      sleeping is possible
> + *
> + * Write lock always allows only one thread to access the data.

<snip>