Re: ll_rw_blk.c fails to merge requests. Help!

From: David Mansfield (
Date: Tue Aug 29 2000 - 15:31:12 EST

> > reqfree-batching-3Name: reqfree-batching-3
> > Type: Plain Text (text/plain)
> This one works even better.
> + list_add(&req->table, &q->pending_freelist);
> + if (++q->pending_free > 64) {
> ^^^^^
I have two comments about this patch. (the reqfree-batching-3)

First. Couldn't it end up indefinitely starving a writer?

Assume the free_list is empty (all requests in use), and the
pending_freelist is empty. (So q->queue_requests ==
QUEUE_NR_REQUESTS). A write request
comes in and is blocked in __get_request_wait. Now 64 ( 64 == 1/4 of
QUEUE_NR_REQUESTS ) requests complete. The pending_freelist is spliced
into the free_list and the wait_queue is woken. Now the writer will
up in __get_request_wait, but will not succeed in get_request so it
go back to sleep (queue_requests == 3/4 of QUEUE_NR_REQUESTS which is >
than 1/2 of QUEUE_NR_REQUESTS). Meanwhile the 64 requests can be used
by a) readers that were blocking, or b) new readers. This could
ad infinitum...

Second. At queue destroy time (blk_cleanup_queue) the pending_list
be spliced into the free_list so they won't get leaked.
add a 'count -= blk_cleanup_queue(&q->pending_list)'.

David Mansfield
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