fs/dcache: Disable preemption on i_dir_seq write side on PREEMPT_RT

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>

diff --git a/fs/dcache.c b/fs/dcache.c
index 94b7373..7196950 100644 (file)
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2564,7 +2564,15 @@ 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)
@@ -2576,6 +2584,8 @@ 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)