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 64b8f737af1b..dc8092e2053a 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -2484,7 +2484,15 @@ 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) @@ -2496,6 +2504,8 @@ 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)