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    Subject[PATCH 4.14 138/496] fs: dcache: Avoid livelock between d_alloc_parallel and __d_add
    Date
    4.14-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Will Deacon <will.deacon@arm.com>

    [ Upstream commit 015555fd4d2930bc0c86952c46ad88b3392f66e4 ]

    If d_alloc_parallel runs concurrently with __d_add, it is possible for
    d_alloc_parallel to continuously retry whilst i_dir_seq has been
    incremented to an odd value by __d_add:

    CPU0:
    __d_add
    n = start_dir_add(dir);
    cmpxchg(&dir->i_dir_seq, n, n + 1) == n

    CPU1:
    d_alloc_parallel
    retry:
    seq = smp_load_acquire(&parent->d_inode->i_dir_seq) & ~1;
    hlist_bl_lock(b);
    bit_spin_lock(0, (unsigned long *)b); // Always succeeds

    CPU0:
    __d_lookup_done(dentry)
    hlist_bl_lock
    bit_spin_lock(0, (unsigned long *)b); // Never succeeds

    CPU1:
    if (unlikely(parent->d_inode->i_dir_seq != seq)) {
    hlist_bl_unlock(b);
    goto retry;
    }

    Since the simple bit_spin_lock used to implement hlist_bl_lock does not
    provide any fairness guarantees, then CPU1 can starve CPU0 of the lock
    and prevent it from reaching end_dir_add(dir), therefore CPU1 cannot
    exit its retry loop because the sequence number always has the bottom
    bit set.

    This patch resolves the livelock by not taking hlist_bl_lock in
    d_alloc_parallel if the sequence counter is odd, since any subsequent
    masked comparison with i_dir_seq will fail anyway.

    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Al Viro <viro@zeniv.linux.org.uk>
    Reported-by: Naresh Madhusudana <naresh.madhusudana@arm.com>
    Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Matthew Wilcox <mawilcox@microsoft.com>
    Signed-off-by: Will Deacon <will.deacon@arm.com>
    Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
    Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    fs/dcache.c | 8 +++++++-
    1 file changed, 7 insertions(+), 1 deletion(-)

    --- a/fs/dcache.c
    +++ b/fs/dcache.c
    @@ -2482,7 +2482,7 @@ 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);
    r_seq = read_seqbegin(&rename_lock);
    dentry = __d_lookup_rcu(parent, name, &d_seq);
    if (unlikely(dentry)) {
    @@ -2503,6 +2503,12 @@ retry:
    rcu_read_unlock();
    goto retry;
    }
    +
    + if (unlikely(seq & 1)) {
    + rcu_read_unlock();
    + goto retry;
    + }
    +
    hlist_bl_lock(b);
    if (unlikely(parent->d_inode->i_dir_seq != seq)) {
    hlist_bl_unlock(b);

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