linux-kernel - [PATCH v3 05/11] md/raid5-cache: Drop RCU usage of conf->log

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Date:   Thu,  2 Jun 2022 12:18:11 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
        Song Liu <song@...nel.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Donald Buczek <buczek@...gen.mpg.de>,
        Guoqing Jiang <guoqing.jiang@...ux.dev>,
        Xiao Ni <xni@...hat.com>, Stephen Bates <sbates@...thlin.com>,
        Martin Oliveira <Martin.Oliveira@...eticom.com>,
        David Sloan <David.Sloan@...eticom.com>,
        Logan Gunthorpe <logang@...tatee.com>
Subject: [PATCH v3 05/11] md/raid5-cache: Drop RCU usage of conf->log

The only place that uses RCU to access conf->log is in
r5l_log_disk_error(). This function is mostly used in the IO path
and once with mddev_lock() held in raid5_change_consistency_policy().

It is known that the IO will be suspended before the log is freed and
r5l_log_exit() is called with the mddev_lock() held.

This should mean that conf->log can not be freed while the function is
being called, so the RCU protection is not necessary. Drop the
rcu_read_lock() as well as the synchronize_rcu() and
rcu_assign_pointer() usage.

Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
---
 drivers/md/raid5-cache.c | 16 +++++-----------
 1 file changed, 5 insertions(+), 11 deletions(-)

diff --git a/drivers/md/raid5-cache.c b/drivers/md/raid5-cache.c
index 69b95005abca..48020d36b334 100644
--- a/drivers/md/raid5-cache.c
+++ b/drivers/md/raid5-cache.c
@@ -1590,18 +1590,13 @@ void r5l_quiesce(struct r5l_log *log, int quiesce)
 
 bool r5l_log_disk_error(struct r5conf *conf)
 {
-	struct r5l_log *log;
-	bool ret;
-	/* don't allow write if journal disk is missing */
-	rcu_read_lock();
-	log = rcu_dereference(conf->log);
+	struct r5l_log *log = conf->log;
 
+	/* don't allow write if journal disk is missing */
 	if (!log)
-		ret = test_bit(MD_HAS_JOURNAL, &conf->mddev->flags);
+		return test_bit(MD_HAS_JOURNAL, &conf->mddev->flags);
 	else
-		ret = test_bit(Faulty, &log->rdev->flags);
-	rcu_read_unlock();
-	return ret;
+		return test_bit(Faulty, &log->rdev->flags);
 }
 
 #define R5L_RECOVERY_PAGE_POOL_SIZE 256
@@ -3148,7 +3143,7 @@ int r5l_init_log(struct r5conf *conf, struct md_rdev *rdev)
 	spin_lock_init(&log->stripe_in_journal_lock);
 	atomic_set(&log->stripe_in_journal_count, 0);
 
-	rcu_assign_pointer(conf->log, log);
+	conf->log = log;
 
 	set_bit(MD_HAS_JOURNAL, &conf->mddev->flags);
 	return 0;
@@ -3173,7 +3168,6 @@ void r5l_exit_log(struct r5conf *conf)
 	lockdep_assert_held(&conf->mddev->reconfig_mutex);
 
 	conf->log = NULL;
-	synchronize_rcu();
 
 	/* Ensure disable_writeback_work wakes up and exits */
 	wake_up(&conf->mddev->sb_wait);
-- 
2.30.2