Date: Mon, 25 Aug 2014 10:15:26 +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: <20140825081526.GB14344@dft-labs.eu> In-Reply-To: <20140825073404.GZ2737@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>
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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>
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