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Message-Id: <20220811161144.1543598-12-sashal@kernel.org>
Date: Thu, 11 Aug 2022 12:11:38 -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 4.9 12/12] 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 e42c715fc9e9..feb592bb7cbe 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2425,7 +2425,15 @@ 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)
@@ -2437,6 +2445,8 @@ 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
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