Commit cf634d54 authored by Sebastian Andrzej Siewior's avatar Sebastian Andrzej Siewior Committed by Al Viro
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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: default avatar <Oleg.Karfich@wago.com>
Signed-off-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lkml.kernel.org/r/20220613140712.77932-2-bigeasy@linutronix.de


Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 40a3cb0d

fs/dcache.c

+11 −1
Original line number Diff line number Diff line
@@ -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)