atomic_load_acq_int in sequential_heuristic

[Mateusz Guzik]

On Mon, Aug 25, 2014 at 10:34:04AM +0300, Konstantin Belousov wrote:
[..]
> Two notes.  First, please add a comment explaining which other part
> of the code is locked against in F_READAHEAD switch case.  Second,
> should the vnode lock cover the FRDAHEAD reset case too, at least
> for consistency ?

I'll start with a side thing:

diff --git a/sys/kern/vfs_vnops.c b/sys/kern/vfs_vnops.c
index 98823f3..2c3df2b 100644
--- a/sys/kern/vfs_vnops.c
+++ b/sys/kern/vfs_vnops.c
@@ -365,7 +365,7 @@ vn_open_vnode(struct vnode *vp, int fmode, struct ucred *cred,
 				fp->f_ops= &badfileops;
 			return (error);
 		}
-		fp->f_flag |= FHASLOCK;
+		atomic_set_int(&fp->f_flag, FHASLOCK);
 	}
 	if (fmode & FWRITE) {
 		VOP_ADD_WRITECOUNT(vp, 1);

And now for the patch in question:

Yes, _rel is not necessary. In fact, the loop is not necessary either. It
would be useful if more than one flag was altered.

diff --git a/sys/kern/kern_descrip.c b/sys/kern/kern_descrip.c
index 7abdca0..433b809 100644
--- a/sys/kern/kern_descrip.c
+++ b/sys/kern/kern_descrip.c
@@ -476,7 +476,6 @@ kern_fcntl(struct thread *td, int fd, int cmd, intptr_t arg)
 	struct vnode *vp;
 	cap_rights_t rights;
 	int error, flg, tmp;
-	u_int old, new;
 	uint64_t bsize;
 	off_t foffset;
 
@@ -762,23 +761,21 @@ kern_fcntl(struct thread *td, int fd, int cmd, intptr_t arg)
 		}
 		if (arg >= 0) {
 			vp = fp->f_vnode;
-			error = vn_lock(vp, LK_SHARED);
+			/*
+			 * Exclusive lock synchronizes against
+			 * sequential_heuristic().
+			 */
+			error = vn_lock(vp, LK_EXCLUSIVE);
 			if (error != 0) {
 				fdrop(fp, td);
 				break;
 			}
 			bsize = fp->f_vnode->v_mount->mnt_stat.f_iosize;
-			VOP_UNLOCK(vp, 0);
 			fp->f_seqcount = (arg + bsize - 1) / bsize;
-			do {
-				new = old = fp->f_flag;
-				new |= FRDAHEAD;
-			} while (!atomic_cmpset_rel_int(&fp->f_flag, old, new));
+			atomic_set_int(&fp->f_flag, FHASLOCK);
+			VOP_UNLOCK(vp, 0);
 		} else {
-			do {
-				new = old = fp->f_flag;
-				new &= ~FRDAHEAD;
-			} while (!atomic_cmpset_rel_int(&fp->f_flag, old, new));
+			atomic_clear_int(&fp->f_flag, FHASLOCK);
 		}
 		fdrop(fp, td);
 		break;
diff --git a/sys/kern/vfs_vnops.c b/sys/kern/vfs_vnops.c
index f1d19ac..98823f3 100644
--- a/sys/kern/vfs_vnops.c
+++ b/sys/kern/vfs_vnops.c
@@ -438,7 +438,8 @@ static int
 sequential_heuristic(struct uio *uio, struct file *fp)
 {
 
-	if (atomic_load_acq_int(&(fp->f_flag)) & FRDAHEAD)
+	ASSERT_VOP_LOCKED(fp->f_vnode, __func__);
+	if (fp->f_flag & FRDAHEAD)
 		return (fp->f_seqcount << IO_SEQSHIFT);
 
 	/*

-- 
Mateusz Guzik <mjguzik gmail.com>