Re: [PATCH v2 11/19] lib/stackdepot: use read/write lock

[Alexander Potapenko <glider@google.com>]

On Wed, Sep 13, 2023 at 7:16 PM <andrey.konovalov@linux.dev> wrote:
>
> From: Andrey Konovalov <andreyknvl@google.com>
>
> Currently, stack depot uses the following locking scheme:
>
> 1. Lock-free accesses when looking up a stack record, which allows to
>    have multiple users to look up records in parallel;
> 2. Spinlock for protecting the stack depot pools and the hash table
>    when adding a new record.
>
> For implementing the eviction of stack traces from stack depot, the
> lock-free approach is not going to work anymore, as we will need to be
> able to also remove records from the hash table.
>
> Convert the spinlock into a read/write lock, and drop the atomic accesses,
> as they are no longer required.
>
> Looking up stack traces is now protected by the read lock and adding new
> records - by the write lock. One of the following patches will add a new
> function for evicting stack records, which will be protected by the write
> lock as well.
>
> With this change, multiple users can still look up records in parallel.
>
> This is preparatory patch for implementing the eviction of stack records
> from the stack depot.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>

Reviewed-by: Alexander Potapenko <glider@google.com>
(but see the comment below)

>  static struct stack_record *depot_fetch_stack(depot_stack_handle_t handle)
>  {
>         union handle_parts parts = { .handle = handle };
> -       /*
> -        * READ_ONCE pairs with potential concurrent write in
> -        * depot_init_pool.
> -        */
> -       int pools_num_cached = READ_ONCE(pools_num);
>         void *pool;
>         size_t offset = parts.offset << DEPOT_STACK_ALIGN;
>         struct stack_record *stack;
>
> -       if (parts.pool_index > pools_num_cached) {
> +       lockdep_assert_held(&pool_rwlock);

Shouldn't it be lockdep_assert_held_read()?