From 12815a7d8f8231f53e075087ed7c6423f8c74d20 Mon Sep 17 00:00:00 2001 From: Gabriel Krisman Bertazi Date: Tue, 15 Nov 2022 17:45:51 -0500 Subject: [PATCH] sbitmap: Advance the queue index before waking up a queue commit 976570b4ecd30d3ec6e1b0910da8e5edc591f2b6 upstream. When a queue is awaken, the wake_index written by sbq_wake_ptr currently keeps pointing to the same queue. On the next wake up, it will thus retry the same queue, which is unfair to other queues, and can lead to starvation. This patch, moves the index update to happen before the queue is returned, such that it will now try a different queue first on the next wake up, improving fairness. Fixes: 4f8126bb2308 ("sbitmap: Use single per-bitmap counting to wake up queued tags") Reported-by: Jan Kara Reviewed-by: Jan Kara Signed-off-by: Gabriel Krisman Bertazi Link: https://lore.kernel.org/r/20221115224553.23594-2-krisman@suse.de Signed-off-by: Jens Axboe Signed-off-by: Greg Kroah-Hartman --- lib/sbitmap.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/lib/sbitmap.c b/lib/sbitmap.c index e918cd8695f1..443accfe685d 100644 --- a/lib/sbitmap.c +++ b/lib/sbitmap.c @@ -566,13 +566,19 @@ static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq) for (i = 0; i < SBQ_WAIT_QUEUES; i++) { struct sbq_wait_state *ws = &sbq->ws[wake_index]; + /* + * Advance the index before checking the current queue. + * It improves fairness, by ensuring the queue doesn't + * need to be fully emptied before trying to wake up + * from the next one. + */ + wake_index = sbq_index_inc(wake_index); + if (waitqueue_active(&ws->wait)) { if (wake_index != atomic_read(&sbq->wake_index)) atomic_set(&sbq->wake_index, wake_index); return ws; } - - wake_index = sbq_index_inc(wake_index); } return NULL;