Date: Mon, 25 Aug 2014 11:10:56 +0200 From: Mateusz Guzik <mjguzik@gmail.com> To: Konstantin Belousov <kostikbel@gmail.com> Cc: freebsd-hackers@freebsd.org Subject: Re: atomic_load_acq_int in sequential_heuristic Message-ID: <20140825091056.GC14344@dft-labs.eu> In-Reply-To: <20140825083539.GB2737@kib.kiev.ua> References: <20140824115729.GC2045@dft-labs.eu> <20140824162331.GW2737@kib.kiev.ua> <20140824164236.GX2737@kib.kiev.ua> <20140825005659.GA14344@dft-labs.eu> <20140825073404.GZ2737@kib.kiev.ua> <20140825081526.GB14344@dft-labs.eu> <20140825083539.GB2737@kib.kiev.ua>
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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>
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