atomic_load_acq_int in sequential_heuristic

[Mateusz Guzik]

On Mon, Aug 25, 2014 at 11:35:39AM +0300, Konstantin Belousov wrote:
> > +			atomic_set_int(&fp->f_flag, FHASLOCK);
> You misspelled FRDAHEAD as FHASLOCK, below as well.
> Was this tested ?
> 

Oops, damn copy-pasto. Sorry.

> > +			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);
> So what about extending the vnode lock to cover the flag reset ?
> 

Sure.

So this time I tested it properly and found out it is impossible to
disable the hint. The test is:

-1 is truncated and then read from intptr_t which yields a big positive
number instead. Other users in the function use int tmp to work around
this issue.

diff --git a/sys/kern/kern_descrip.c b/sys/kern/kern_descrip.c
index 7abdca0..774f647 100644
--- a/sys/kern/kern_descrip.c
+++ b/sys/kern/kern_descrip.c
@@ -760,7 +760,8 @@ kern_fcntl(struct thread *td, int fd, int cmd, intptr_t arg)
 			error = EBADF;
 			break;
 		}
-		if (arg >= 0) {
+		tmp = arg;
+		if (tmp >= 0) {
 			vp = fp->f_vnode;
 			error = vn_lock(vp, LK_SHARED);
 			if (error != 0) {
@@ -769,7 +770,7 @@ kern_fcntl(struct thread *td, int fd, int cmd, intptr_t arg)
 			}
 			bsize = fp->f_vnode->v_mount->mnt_stat.f_iosize;
 			VOP_UNLOCK(vp, 0);
-			fp->f_seqcount = (arg + bsize - 1) / bsize;
+			fp->f_seqcount = (tmp + bsize - 1) / bsize;
 			do {
 				new = old = fp->f_flag;
 				new |= FRDAHEAD;

Then the patch in question:

diff --git a/sys/kern/kern_descrip.c b/sys/kern/kern_descrip.c
index 774f647..4efadb0 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;
 
@@ -760,27 +759,25 @@ kern_fcntl(struct thread *td, int fd, int cmd, intptr_t arg)
 			error = EBADF;
 			break;
 		}
+		vp = fp->f_vnode;
+		/*
+		 * Exclusive lock synchronizes against
+		 * sequential_heuristic().
+		 */
+		error = vn_lock(vp, LK_EXCLUSIVE);
+		if (error != 0) {
+			fdrop(fp, td);
+			break;
+		}
 		tmp = arg;
 		if (tmp >= 0) {
-			vp = fp->f_vnode;
-			error = vn_lock(vp, LK_SHARED);
-			if (error != 0) {
-				fdrop(fp, td);
-				break;
-			}
 			bsize = fp->f_vnode->v_mount->mnt_stat.f_iosize;
-			VOP_UNLOCK(vp, 0);
 			fp->f_seqcount = (tmp + 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, FRDAHEAD);
 		} 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, FRDAHEAD);
 		}
+		VOP_UNLOCK(vp, 0);
 		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>