nfs: avoid i_lock contention in nfs_clear_invalid_mapping

Multi-threaded buffered reads to the same file exposed significant
inode spinlock contention in nfs_clear_invalid_mapping().

Eliminate this spinlock contention by checking flags without locking,
instead using smp_rmb and smp_load_acquire accordingly, but then take
spinlock and double-check these inode flags.

Also refactor nfs_set_cache_invalid() slightly to use
smp_store_release() to pair with nfs_clear_invalid_mapping()'s
smp_load_acquire().

While this fix is beneficial for all multi-threaded buffered reads
issued by an NFS client, this issue was identified in the context of
surprisingly low LOCALIO performance with 4K multi-threaded buffered
read IO. This fix dramatically speeds up LOCALIO performance:

before: read: IOPS=1583k, BW=6182MiB/s (6482MB/s)(121GiB/20002msec)
after: read: IOPS=3046k, BW=11.6GiB/s (12.5GB/s)(232GiB/20001msec)

Fixes: 17dfeb911339 ("NFS: Fix races in nfs_revalidate_mapping")
Signed-off-by: Mike Snitzer <[e-mail]>
Reviewed-by: Jeff Layton <[e-mail]>
Signed-off-by: Anna Schumaker <[e-mail]>

fs/nfs/inode.c

···
 		nfs_fscache_invalidate(inode, 0);
 	flags &= ~NFS_INO_REVAL_FORCED;
 
-	nfsi->cache_validity |= flags;
+	flags |= nfsi->cache_validity;
+	if (inode->i_mapping->nrpages == 0)
+		flags &= ~NFS_INO_INVALID_DATA;
 
-	if (inode->i_mapping->nrpages == 0) {
-		nfsi->cache_validity &= ~NFS_INO_INVALID_DATA;
-		nfs_ooo_clear(nfsi);
-	} else if (nfsi->cache_validity & NFS_INO_INVALID_DATA) {
+	/* pairs with nfs_clear_invalid_mapping()'s smp_load_acquire() */
+	smp_store_release(&nfsi->cache_validity, flags);
+
+	if (inode->i_mapping->nrpages == 0 ||
+	    nfsi->cache_validity & NFS_INO_INVALID_DATA) {
 		nfs_ooo_clear(nfsi);
 	}
 	trace_nfs_set_cache_invalid(inode, 0);
···
 					 TASK_KILLABLE|TASK_FREEZABLE_UNSAFE);
 		if (ret)
 			goto out;
+		smp_rmb(); /* pairs with smp_wmb() below */
+		if (test_bit(NFS_INO_INVALIDATING, bitlock))
+			continue;
+		/* pairs with nfs_set_cache_invalid()'s smp_store_release() */
+		if (!(smp_load_acquire(&nfsi->cache_validity) & NFS_INO_INVALID_DATA))
+			goto out;
+		/* Slow-path that double-checks with spinlock held */
 		spin_lock(&inode->i_lock);
 		if (test_bit(NFS_INO_INVALIDATING, bitlock)) {
 			spin_unlock(&inode->i_lock);