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

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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>
This commit is contained in:
Sebastian Andrzej Siewior 2022-07-27 13:49:02 +02:00 committed by Al Viro
parent 40a3cb0d23
commit cf634d540a
1 changed files with 11 additions and 1 deletions
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12
fs/dcache.c
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@ -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)