Re: [PATCH 1/6] block: add blk_io_plug_call() API

[Eric Blake]

On Wed, May 17, 2023 at 06:10:17PM -0400, Stefan Hajnoczi wrote:
> Introduce a new API for thread-local blk_io_plug() that does not
> traverse the block graph. The goal is to make blk_io_plug() multi-queue
> friendly.
> 
> Instead of having block drivers track whether or not we're in a plugged
> section, provide an API that allows them to defer a function call until
> we're unplugged: blk_io_plug_call(fn, opaque). If blk_io_plug_call() is
> called multiple times with the same fn/opaque pair, then fn() is only
> called once at the end of the function - resulting in batching.
> 
> This patch introduces the API and changes blk_io_plug()/blk_io_unplug().
> blk_io_plug()/blk_io_unplug() no longer require a BlockBackend argument
> because the plug state is now thread-local.
> 
> Later patches convert block drivers to blk_io_plug_call() and then we
> can finally remove .bdrv_co_io_plug() once all block drivers have been
> converted.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---

> +++ b/block/plug.c
> +
> +/**
> + * blk_io_plug_call:
> + * @fn: a function pointer to be invoked
> + * @opaque: a user-defined argument to @fn()
> + *
> + * Call @fn(@opaque) immediately if not within a 
> blk_io_plug()/blk_io_unplug()
> + * section.
> + *
> + * Otherwise defer the call until the end of the outermost
> + * blk_io_plug()/blk_io_unplug() section in this thread. If the same
> + * @fn/@opaque pair has already been deferred, it will only be called once 
> upon
> + * blk_io_unplug() so that accumulated calls are batched into a single call.
> + *
> + * The caller must ensure that @opaque is not be freed before @fn() is 
> invoked.

s/be //

> + */
> +void blk_io_plug_call(void (*fn)(void *), void *opaque)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
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