Re: [RFC PATCH 1/2] kunit: resource: Add kunit_defer() functionality

[Maxime Ripard <maxime@cerno.tech>]

Hi David,

Thanks a lot for doing this

On Sat, Mar 25, 2023 at 12:31:03PM +0800, David Gow wrote:
> Many uses of the KUnit resource system are intended to simply defer
> calling a function until the test exits (be it due to success or
> failure). The existing kunit_alloc_resource() function is often used for
> this, but was awkward to use (requiring passing NULL init functions, etc),
> and returned a resource without incrementing its reference count, which
> -- while okay for this use-case -- could cause problems in others.
> 
> Instead, introduce a simple kunit_defer() API: a simple function
> (returning nothing, accepting a single void* argument) can be scheduled
> to be called when the test exits. Deferred functions are called in the
> opposite order to that which they were registered.
> 
> This is implemented as a resource under the hood, so the ordering
> between resource cleanup and deferred functions is maintained.
> 
> Signed-off-by: David Gow <davidgow@google.com>
> ---
>  include/kunit/resource.h |  87 +++++++++++++++++++++++++++++++
>  lib/kunit/resource.c     | 110 +++++++++++++++++++++++++++++++++++++++
>  2 files changed, 197 insertions(+)
> 
> diff --git a/include/kunit/resource.h b/include/kunit/resource.h
> index cf6fb8f2ac1b..6c4728ca9237 100644
> --- a/include/kunit/resource.h
> +++ b/include/kunit/resource.h
> @@ -387,4 +387,91 @@ static inline int kunit_destroy_named_resource(struct kunit *test,
>   */
>  void kunit_remove_resource(struct kunit *test, struct kunit_resource *res);
>  
> +typedef void (*kunit_defer_function_t)(void *ctx);
> +
> +/**
> + * kunit_defer() - Defer a function call until the test ends.

devm (and drm resource management) uses devm_add_action for this, and
providing consistency across resource management API would be neat.

> + * @test: Test case to associate the deferred function with.
> + * @func: The function to run on test exit
> + * @ctx: Data passed into @func
> + * @internal_gfp: gfp to use for internal allocations, if unsure, use GFP_KERNEL

Do you really expect any GFP flag other than GFP_KERNEL?

> + * Defer the execution of a function until the test exits, either normally or
> + * due to a failure.  @ctx is passed as additional context. All functions
> + * registered with kunit_defer() will execute in the opposite order to that
> + * they were registered in.
> + *
> + * This is useful for cleaning up allocated memory and resources.
> + *
> + * RETURNS:
> + *   An opaque "cancellation token", or NULL on error. Pass this token to
> + *   kunit_defer_cancel() in order to cancel the deferred execution of func().
> + */

devm also gets away with this by (iirc) associating the function
pointer and context to the device. That triplet is pretty much
guaranteed to be unique across calls and thus you don't have to keep
(and pass!) a token around.

> +void *kunit_defer(struct kunit *test, kunit_defer_function_t func,
> +		  void *ctx, gfp_t internal_gfp);
> +
> +/**
> + * kunit_defer_cancel_token() - Cancel a deferred function call.
> + * @test: Test case the deferred function is associated with.
> + * @cancel_token: The cancellation token returned by kunit_defer()
> + *
> + * Prevent a function deferred using kunit_defer() from executing when the
> + * test ends.
> + *
> + * Prefer using this to kunit_defer_cancel() where possible.
> + */
> +void kunit_defer_cancel_token(struct kunit *test, void *cancel_token);
> +
> +/**
> + * kunit_defer_trigger_token() - Run a deferred function call immediately.
> + * @test: Test case the deferred function is associated with.
> + * @cancel_token: The cancellation token returned by kunit_defer()
> + *
> + * Execute a deferred function call immediately, instead of waiting for the
> + * test to end.
> + *
> + * Prefer using this to kunit_defer_trigger() where possible.
> + */
> +void kunit_defer_trigger_token(struct kunit *test, void *cancel_token);
> +
> +/**
> + * kunit_defer_cancel() - Cancel a matching deferred function call.
> + * @test: Test case the deferred function is associated with.
> + * @func: The deferred function to cancel.
> + * @ctx: The context passed to the deferred function to trigger.
> + *
> + * Prevent a function deferred via kunit_defer() from executing at shutdown.
> + * Unlike kunit_defer_cancel_token(), this takes the (func, ctx) pair instead of
> + * the cancellation token. If that function/context pair was deferred multiple
> + * times, only the most recent one will be cancelled.
> + *
> + * Prefer using kunit_defer_cancel_token() to this where possible.
> + */
> +void kunit_defer_cancel(struct kunit *test,
> +			kunit_defer_function_t func,
> +			void *ctx);
> +
> +/**
> + * kunit_defer_trigger() - Run a matching deferred function call immediately.
> + * @test: Test case the deferred function is associated with.
> + * @func: The deferred function to trigger.
> + * @ctx: The context passed to the deferred function to trigger.
> + *
> + * Execute a function deferred via kunit_defer() immediately, rather than when
> + * the test ends.
> + * Unlike kunit_defer_trigger_token(), this takes the (func, ctx) pair instead of
> + * the cancellation token. If that function/context pair was deferred multiple
> + * times, it will only be executed once here. The most recent deferral will
> + * no longer execute when the test ends.
> + *
> + * kunit_defer_trigger(test, func, ctx);
> + * is equivalent to
> + * func(ctx);
> + * kunit_defer_cancel(test, func, ctx);
> + *
> + * Prefer using kunit_defer_trigger_token() to this where possible.
> + */
> +void kunit_defer_trigger(struct kunit *test,
> +			 kunit_defer_function_t func,
> +			 void *ctx);
>  #endif /* _KUNIT_RESOURCE_H */

I think I agree with Matti here, you might want some function to
cancel an action/deferral but it's not clear to me why you would need
all the other functions there (for now at least)

Maxime