quota: Fix possible races during quota loading

When loading new quota structure from disk, there is a possibility caller
of dqget() will see uninitialized data due to CPU reordering loads or
stores - loads from dquot can be reordered before test of DQ_ACTIVE_B
bit or setting of this bit could be reordered before filling of the
structure. Fix the issue by adding proper memory barriers.

Signed-off-by: Jan Kara <jack@suse.cz>

diff --git a/fs/quota/dquot.c b/fs/quota/dquot.c
index 7e0137b..e8467dc 100644 (file)
--- a/fs/quota/dquot.c
+++ b/fs/quota/dquot.c
@@ -411,6 +411,8 @@ int dquot_acquire(struct dquot *dquot)
                ret = dqopt->ops[dquot->dq_id.type]->read_dqblk(dquot);
        if (ret < 0)
                goto out_iolock;
+       /* Make sure flags update is visible after dquot has been filled */
+       smp_mb__before_atomic();
        set_bit(DQ_READ_B, &dquot->dq_flags);
        /* Instantiate dquot if needed */
        if (!test_bit(DQ_ACTIVE_B, &dquot->dq_flags) && !dquot->dq_off) {
@@ -427,6 +429,11 @@ int dquot_acquire(struct dquot *dquot)
                        goto out_iolock;
                }
        }
+       /*
+        * Make sure flags update is visible after on-disk struct has been
+        * allocated. Paired with smp_rmb() in dqget().
+        */
+       smp_mb__before_atomic();
        set_bit(DQ_ACTIVE_B, &dquot->dq_flags);
 out_iolock:
        mutex_unlock(&dqopt->dqio_mutex);
@@ -887,6 +894,11 @@ we_slept:
                        goto out;
                }
        }
+       /*
+        * Make sure following reads see filled structure - paired with
+        * smp_mb__before_atomic() in dquot_acquire().
+        */
+       smp_rmb();
 #ifdef CONFIG_QUOTA_DEBUG
        BUG_ON(!dquot->dq_sb);  /* Has somebody invalidated entry under us? */
 #endif