[PATCH AUTOSEL 4.19 13/14] d_add_ci(): make sure we don't miss d_lookup_done()

[PATCH AUTOSEL 4.19 13/14] d_add_ci(): make sure we don't miss d_lookup_done()

[Sasha Levin]

From: Al Viro <viro@zeniv.linux.org.uk>

[ Upstream commit 40a3cb0d2314a41975aa385a74643878454f6eac ]

All callers of d_alloc_parallel() must make sure that resulting
in-lookup dentry (if any) will encounter __d_lookup_done() before
the final dput().  d_add_ci() might end up creating in-lookup
dentries; they are fed to d_splice_alias(), which will normally
make sure they meet __d_lookup_done().  However, it is possible
to end up with d_splice_alias() failing with ERR_PTR(-ELOOP)
without having done so.  It takes a corrupted ntfs or case-insensitive
xfs image, but neither should end up with memory corruption...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/dcache.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/dcache.c b/fs/dcache.c
index 1897833a4668..1e9f4dd94e6c 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2080,6 +2080,7 @@ struct dentry *d_add_ci(struct dentry *dentry, struct inode *inode,
 	}
 	res = d_splice_alias(inode, found);
 	if (res) {
+		d_lookup_done(found);
 		dput(found);
 		return res;
 	}
-- 
2.35.1

[PATCH AUTOSEL 4.19 14/14] fs/dcache: Disable preemption on i_dir_seq write side on PREEMPT_RT

[Sasha Levin]

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

diff --git a/fs/dcache.c b/fs/dcache.c
index 1e9f4dd94e6c..6f6a972ea20c 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2406,7 +2406,15 @@ 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)
@@ -2418,6 +2426,8 @@ 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

Re: [PATCH AUTOSEL 4.19 14/14] fs/dcache: Disable preemption on i_dir_seq write side on PREEMPT_RT

[Pavel Machek]

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Hi!

> From: Sebastian Andrzej Siewior <bigeasy@linutronix.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

This should not be applied to 4.19 as CONFIG_PREEMPT_RT does not exist
there.

Best regards,
								Pavel
-- 
 'DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk'
 'HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany'


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