linux-kernel - Re: [PATCH 1/5] md/raid5: Refactor raid5_get_active

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Message-ID: <YuKZmloAcZWY5of8@infradead.org>
Date:   Thu, 28 Jul 2022 07:13:46 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
        Song Liu <song@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Guoqing Jiang <guoqing.jiang@...ux.dev>,
        Stephen Bates <sbates@...thlin.com>,
        Martin Oliveira <Martin.Oliveira@...eticom.com>,
        David Sloan <David.Sloan@...eticom.com>
Subject: Re: [PATCH 1/5] md/raid5: Refactor raid5_get_active_stripe()

On Wed, Jul 27, 2022 at 03:05:56PM -0600, Logan Gunthorpe wrote:
> Refactor the raid5_get_active_stripe() to read more linearly in
> the order it's typically executed.
> 
> The init_stripe() call is called if a free stripe is found and the
> function is exited early which removes a lot of if (sh) checks and
> unindents the following code.
> 
> Remove the while loop in favour of the 'goto retry' pattern, which
> reduces indentation further. And use a 'goto wait_for_stripe' instead
> of an additional indent seeing it is the unusual path and this makes
> the code easier to read.
> 
> No functional changes intended. Will make subsequent changes
> in patches easier to understand.

I find the new loop even more confusing than the old one.  I'd go
with something like the version below (on top of the whol md-next tree
that pulled this in way too fast..)

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 4456ac51f7c53..cd8ec4995a49b 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -811,54 +811,54 @@ static struct stripe_head *__raid5_get_active_stripe(struct r5conf *conf,
 
 	spin_lock_irq(conf->hash_locks + hash);
 
-retry:
-	if (!noquiesce && conf->quiesce) {
-		/*
-		 * Must release the reference to batch_last before waiting,
-		 * on quiesce, otherwise the batch_last will hold a reference
-		 * to a stripe and raid5_quiesce() will deadlock waiting for
-		 * active_stripes to go to zero.
-		 */
-		if (ctx && ctx->batch_last) {
-			raid5_release_stripe(ctx->batch_last);
-			ctx->batch_last = NULL;
-		}
-
-		wait_event_lock_irq(conf->wait_for_quiescent, !conf->quiesce,
-				    *(conf->hash_locks + hash));
-	}
+	for (;;) {
+		if (!noquiesce && conf->quiesce) {
+			/*
+			 * Must release the reference to batch_last before
+			 * waiting on quiesce, otherwise the batch_last will
+			 * hold a reference to a stripe and raid5_quiesce()
+			 * will deadlock waiting for active_stripes to go to
+			 * zero.
+			 */
+			if (ctx && ctx->batch_last) {
+				raid5_release_stripe(ctx->batch_last);
+				ctx->batch_last = NULL;
+			}
 
-	sh = find_get_stripe(conf, sector, conf->generation - previous, hash);
-	if (sh)
-		goto out;
+			wait_event_lock_irq(conf->wait_for_quiescent,
+					    !conf->quiesce,
+					    *(conf->hash_locks + hash));
+		}
 
-	if (test_bit(R5_INACTIVE_BLOCKED, &conf->cache_state))
-		goto wait_for_stripe;
+		sh = find_get_stripe(conf, sector, conf->generation - previous,
+				     hash);
+		if (sh)
+			break;
 
-	sh = get_free_stripe(conf, hash);
-	if (sh) {
-		r5c_check_stripe_cache_usage(conf);
-		init_stripe(sh, sector, previous);
-		atomic_inc(&sh->count);
-		goto out;
-	}
+		if (!test_bit(R5_INACTIVE_BLOCKED, &conf->cache_state)) {
+			sh = get_free_stripe(conf, hash);
+			if (sh) {
+				r5c_check_stripe_cache_usage(conf);
+				init_stripe(sh, sector, previous);
+				atomic_inc(&sh->count);
+				break;
+			}
 
-	if (!test_bit(R5_DID_ALLOC, &conf->cache_state))
-		set_bit(R5_ALLOC_MORE, &conf->cache_state);
+			if (!test_bit(R5_DID_ALLOC, &conf->cache_state))
+				set_bit(R5_ALLOC_MORE, &conf->cache_state);
+		}
 
-wait_for_stripe:
-	if (noblock)
-		goto out;
+		if (noblock)
+			break;
 
-	set_bit(R5_INACTIVE_BLOCKED, &conf->cache_state);
-	r5l_wake_reclaim(conf->log, 0);
-	wait_event_lock_irq(conf->wait_for_stripe,
-			    is_inactive_blocked(conf, hash),
-			    *(conf->hash_locks + hash));
-	clear_bit(R5_INACTIVE_BLOCKED, &conf->cache_state);
-	goto retry;
+		set_bit(R5_INACTIVE_BLOCKED, &conf->cache_state);
+		r5l_wake_reclaim(conf->log, 0);
+		wait_event_lock_irq(conf->wait_for_stripe,
+				    is_inactive_blocked(conf, hash),
+				    *(conf->hash_locks + hash));
+		clear_bit(R5_INACTIVE_BLOCKED, &conf->cache_state);
+	}
 
-out:
 	spin_unlock_irq(conf->hash_locks + hash);
 	return sh;
 }