linux-kernel - [PATCH AUTOSEL 4.14 14/14] fs/dcache: Disable preemption on i_dir_seq write side on PREEMPT

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Date:   Thu, 11 Aug 2022 12:10:43 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleg.Karfich@...o.com, Thomas Gleixner <tglx@...utronix.de>,
        Al Viro <viro@...iv.linux.org.uk>,
        Sasha Levin <sashal@...nel.org>, linux-fsdevel@...r.kernel.org
Subject: [PATCH AUTOSEL 4.14 14/14] fs/dcache: Disable preemption on i_dir_seq write side on PREEMPT_RT

From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

[ Upstream commit cf634d540a29018e8d69ab1befb7e08182bc6594 ]

i_dir_seq is a sequence counter with a lock which is represented by the
lowest bit. The writer atomically updates the counter which ensures that it
can be modified by only one writer at a time. This requires preemption to
be disabled across the write side critical section.

On !PREEMPT_RT kernels this is implicit by the caller acquiring
dentry::lock. On PREEMPT_RT kernels spin_lock() does not disable preemption
which means that a preempting writer or reader would live lock. It's
therefore required to disable preemption explicitly.

An alternative solution would be to replace i_dir_seq with a seqlock_t for
PREEMPT_RT, but that comes with its own set of problems due to arbitrary
lock nesting. A pure sequence count with an associated spinlock is not
possible because the locks held by the caller are not necessarily related.

As the critical section is small, disabling preemption is a sensible
solution.

Reported-by: Oleg.Karfich@...o.com
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Link: https://lkml.kernel.org/r/20220613140712.77932-2-bigeasy@linutronix.de
Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 fs/dcache.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index e12246378834..faf6ca33fd51 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2455,7 +2455,15 @@ EXPORT_SYMBOL(d_rehash);
 
 static inline unsigned start_dir_add(struct inode *dir)
 {
-
+	/*
+	 * The caller holds a spinlock (dentry::d_lock). On !PREEMPT_RT
+	 * kernels spin_lock() implicitly disables preemption, but not on
+	 * PREEMPT_RT.  So for RT it has to be done explicitly to protect
+	 * the sequence count write side critical section against a reader
+	 * or another writer preempting, which would result in a live lock.
+	 */
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_disable();
 	for (;;) {
 		unsigned n = dir->i_dir_seq;
 		if (!(n & 1) && cmpxchg(&dir->i_dir_seq, n, n + 1) == n)
@@ -2467,6 +2475,8 @@ static inline unsigned start_dir_add(struct inode *dir)
 static inline void end_dir_add(struct inode *dir, unsigned n)
 {
 	smp_store_release(&dir->i_dir_seq, n + 2);
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_enable();
 }
 
 static void d_wait_lookup(struct dentry *dentry)
-- 
2.35.1