i_dir_seq is an opencoded seqcounter. Based on the code it looks like we could have two writers in parallel despite the fact that the d_lock is held. The problem is that during the write process on RT the preemption is still enabled and if this process is interrupted by a reader with RT priority then we lock up. To avoid that lock up I am disabling the preemption during the update. The rename of i_dir_seq is here to ensure to catch new write sides in future.
Cc: [email protected] Reported-by: [email protected] Signed-off-by: Sebastian Andrzej Siewior <[email protected]> --- fs/dcache.c | 12 +++++++----- fs/inode.c | 2 +- fs/libfs.c | 6 ++++-- include/linux/fs.h | 2 +- 4 files changed, 13 insertions(+), 9 deletions(-) --- a/fs/dcache.c +++ b/fs/dcache.c @@ -2405,9 +2405,10 @@ EXPORT_SYMBOL(d_rehash); static inline unsigned start_dir_add(struct inode *dir) { + preempt_disable_rt(); for (;;) { - unsigned n = dir->i_dir_seq; - if (!(n & 1) && cmpxchg(&dir->i_dir_seq, n, n + 1) == n) + unsigned n = dir->__i_dir_seq; + if (!(n & 1) && cmpxchg(&dir->__i_dir_seq, n, n + 1) == n) return n; cpu_relax(); } @@ -2415,7 +2416,8 @@ static inline unsigned start_dir_add(str static inline void end_dir_add(struct inode *dir, unsigned n) { - smp_store_release(&dir->i_dir_seq, n + 2); + smp_store_release(&dir->__i_dir_seq, n + 2); + preempt_enable_rt(); } static void d_wait_lookup(struct dentry *dentry) @@ -2448,7 +2450,7 @@ struct dentry *d_alloc_parallel(struct d retry: rcu_read_lock(); - seq = smp_load_acquire(&parent->d_inode->i_dir_seq) & ~1; + seq = smp_load_acquire(&parent->d_inode->__i_dir_seq) & ~1; r_seq = read_seqbegin(&rename_lock); dentry = __d_lookup_rcu(parent, name, &d_seq); if (unlikely(dentry)) { @@ -2470,7 +2472,7 @@ struct dentry *d_alloc_parallel(struct d goto retry; } hlist_bl_lock(b); - if (unlikely(parent->d_inode->i_dir_seq != seq)) { + if (unlikely(parent->d_inode->__i_dir_seq != seq)) { hlist_bl_unlock(b); rcu_read_unlock(); goto retry; --- a/fs/inode.c +++ b/fs/inode.c @@ -154,7 +154,7 @@ int inode_init_always(struct super_block inode->i_bdev = NULL; inode->i_cdev = NULL; inode->i_link = NULL; - inode->i_dir_seq = 0; + inode->__i_dir_seq = 0; inode->i_rdev = 0; inode->dirtied_when = 0; --- a/fs/libfs.c +++ b/fs/libfs.c @@ -90,7 +90,7 @@ static struct dentry *next_positive(stru struct list_head *from, int count) { - unsigned *seq = &parent->d_inode->i_dir_seq, n; + unsigned *seq = &parent->d_inode->__i_dir_seq, n; struct dentry *res; struct list_head *p; bool skipped; @@ -123,8 +123,9 @@ static struct dentry *next_positive(stru static void move_cursor(struct dentry *cursor, struct list_head *after) { struct dentry *parent = cursor->d_parent; - unsigned n, *seq = &parent->d_inode->i_dir_seq; + unsigned n, *seq = &parent->d_inode->__i_dir_seq; spin_lock(&parent->d_lock); + preempt_disable_rt(); for (;;) { n = *seq; if (!(n & 1) && cmpxchg(seq, n, n + 1) == n) @@ -137,6 +138,7 @@ static void move_cursor(struct dentry *c else list_add_tail(&cursor->d_child, &parent->d_subdirs); smp_store_release(seq, n + 2); + preempt_enable_rt(); spin_unlock(&parent->d_lock); } --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -651,7 +651,7 @@ struct inode { struct block_device *i_bdev; struct cdev *i_cdev; char *i_link; - unsigned i_dir_seq; + unsigned __i_dir_seq; }; __u32 i_generation;
