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From 44eeab3501a8354ba4c3fe6414168df6ccf60b74 Mon Sep 17 00:00:00 2001
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Fri, 20 Oct 2017 11:29:53 +0200
Subject: [PATCH 172/347] fs/dcache: disable preemption on i_dir_seq's write
side
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.19/older/patches-4.19.246-rt110.tar.xz
i_dir_seq is an opencoded seqcounter. Based on the code it looks like we
could have two writers in parallel despite the fact that the d_lock is
held. The problem is that during the write process on RT the preemption
is still enabled and if this process is interrupted by a reader with RT
priority then we lock up.
To avoid that lock up I am disabling the preemption during the update.
The rename of i_dir_seq is here to ensure to catch new write sides in
future.
Cc: stable-rt@vger.kernel.org
Reported-by: Oleg.Karfich@wago.com
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
fs/dcache.c | 12 +++++++-----
fs/inode.c | 2 +-
include/linux/fs.h | 2 +-
3 files changed, 9 insertions(+), 7 deletions(-)
diff --git a/fs/dcache.c b/fs/dcache.c
index b5e53587d14b..1ae163d54925 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2406,9 +2406,10 @@ EXPORT_SYMBOL(d_rehash);
static inline unsigned start_dir_add(struct inode *dir)
{
+ preempt_disable_rt();
for (;;) {
- unsigned n = dir->i_dir_seq;
- if (!(n & 1) && cmpxchg(&dir->i_dir_seq, n, n + 1) == n)
+ unsigned n = dir->__i_dir_seq;
+ if (!(n & 1) && cmpxchg(&dir->__i_dir_seq, n, n + 1) == n)
return n;
cpu_relax();
}
@@ -2416,7 +2417,8 @@ 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);
+ smp_store_release(&dir->__i_dir_seq, n + 2);
+ preempt_enable_rt();
}
static void d_wait_lookup(struct dentry *dentry)
@@ -2449,7 +2451,7 @@ struct dentry *d_alloc_parallel(struct dentry *parent,
retry:
rcu_read_lock();
- seq = smp_load_acquire(&parent->d_inode->i_dir_seq);
+ seq = smp_load_acquire(&parent->d_inode->__i_dir_seq);
r_seq = read_seqbegin(&rename_lock);
dentry = __d_lookup_rcu(parent, name, &d_seq);
if (unlikely(dentry)) {
@@ -2477,7 +2479,7 @@ struct dentry *d_alloc_parallel(struct dentry *parent,
}
hlist_bl_lock(b);
- if (unlikely(READ_ONCE(parent->d_inode->i_dir_seq) != seq)) {
+ if (unlikely(READ_ONCE(parent->d_inode->__i_dir_seq) != seq)) {
hlist_bl_unlock(b);
rcu_read_unlock();
goto retry;
diff --git a/fs/inode.c b/fs/inode.c
index c9eb5041ffae..e1fb0b743c96 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -156,7 +156,7 @@ int inode_init_always(struct super_block *sb, struct inode *inode)
inode->i_bdev = NULL;
inode->i_cdev = NULL;
inode->i_link = NULL;
- inode->i_dir_seq = 0;
+ inode->__i_dir_seq = 0;
inode->i_rdev = 0;
inode->dirtied_when = 0;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 86f884e78b6b..029870aca4ff 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -679,7 +679,7 @@ struct inode {
struct block_device *i_bdev;
struct cdev *i_cdev;
char *i_link;
- unsigned i_dir_seq;
+ unsigned __i_dir_seq;
};
__u32 i_generation;
--
2.36.1
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