On Tue, 20 Jun 2006, Andrew Morton wrote:
> On Wed, 21 Jun 2006 14:18:15 +0800
> Ian Kent <[email protected]> wrote:
>
> >
> > Hi Andrew,
> >
> > I didn't get any adverse (or other feedback) for this patch after posting
> > an RFC to LKML so here it is.
> >
> > For a long time now I have had a problem with not being able to return a
> > lookup failure on an existsing directory. In autofs this corresponds to a
> > mount failure on a autofs managed mount entry that is browsable (and so
> > the mount point directory exists).
> >
> > While this problem has been present for a long time I've avoided resolving
> > it because it was not very visible. But now that autofs v5 has "mount and
> > expire on demand" of nested multiple mounts, such as is found when
> > mounting an export list from a server, solving the problem cannot be
> > avoided any longer.
> >
> > I've tried very hard to find a way to do this entirely within the
> > autofs4 module but have not been able to find a satisfactory way to
> > achieve it.
> >
> > So, I need to propose a change to the VFS.
> >
> > --- linux-2.6.17/fs/namei.c.dcache-revalidate-return-fail 2006-06-19 13:26:27.000000000 +0800
> > +++ linux-2.6.17/fs/namei.c 2006-06-19 13:31:31.000000000 +0800
> > @@ -380,9 +380,24 @@ static struct dentry * cached_lookup(str
> > dentry = d_lookup(parent, name);
> >
> > if (dentry && dentry->d_op && dentry->d_op->d_revalidate) {
> > - if (!dentry->d_op->d_revalidate(dentry, nd) && !d_invalidate(dentry)) {
> > - dput(dentry);
> > - dentry = NULL;
> > + if (!dentry->d_op->d_revalidate(dentry, nd)) {
> > + if (!d_invalidate(dentry)) {
> > + dput(dentry);
> > + return NULL;
> > + }
> > + /*
> > + * As well as the normal validation, check if we need
> > + * to force a fail on a valid dentry (autofs4 browsable
> > + * mounts).
> > + */
> > + spin_lock(&dentry->d_lock);
> > + if (dentry->d_flags & DCACHE_REVAL_FORCE_FAIL) {
> > + dentry->d_flags &= ~DCACHE_REVAL_FORCE_FAIL;
> > + spin_unlock(&dentry->d_lock);
> > + dput(dentry);
> > + return ERR_PTR(-ENOENT);
> > + }
> > + spin_unlock(&dentry->d_lock);
> > }
> > }
> > return dentry;
> > @@ -477,9 +492,24 @@ static struct dentry * real_lookup(struc
> > */
> > mutex_unlock(&dir->i_mutex);
> > if (result->d_op && result->d_op->d_revalidate) {
> > - if (!result->d_op->d_revalidate(result, nd) && !d_invalidate(result)) {
> > - dput(result);
> > - result = ERR_PTR(-ENOENT);
> > + if (!result->d_op->d_revalidate(result, nd)) {
> > + if (!d_invalidate(result)) {
> > + dput(result);
> > + return ERR_PTR(-ENOENT);
> > + }
> > + /*
> > + * d_revalidate failed but the dentry is still valid so
> > + * check if we need to force a fail on the dentry (autofs4
> > + * browsable mounts).
> > + */
> > + spin_lock(&result->d_lock);
> > + if (result->d_flags & DCACHE_REVAL_FORCE_FAIL) {
> > + result->d_flags &= ~DCACHE_REVAL_FORCE_FAIL;
> > + spin_unlock(&result->d_lock);
> > + dput(result);
> > + return ERR_PTR(-ENOENT);
> > + }
> > + spin_unlock(&result->d_lock);
> > }
> > }
> > return result;
> > @@ -762,8 +792,21 @@ need_lookup:
> > need_revalidate:
> > if (dentry->d_op->d_revalidate(dentry, nd))
> > goto done;
> > - if (d_invalidate(dentry))
> > + if (d_invalidate(dentry)) {
> > + /*
> > + * d_revalidate failed but the dentry is still valid so check
> > + * if we need to return a fail (autofs4 browsable mounts).
> > + */
> > + spin_lock(&dentry->d_lock);
> > + if (dentry->d_flags & DCACHE_REVAL_FORCE_FAIL) {
> > + dentry->d_flags &= ~DCACHE_REVAL_FORCE_FAIL;
> > + spin_unlock(&dentry->d_lock);
> > + dput(dentry);
> > + return -ENOENT;
> > + }
> > + spin_unlock(&dentry->d_lock);
> > goto done;
> > + }
>
> All these are basically the same. Could you look at creating a common
> function, please?
Yep, I'll do that.
Jeff suggested that when he saw it.
>
> Also, I don't know how frequently that code path is executed (presumably
> infrequently) but would the semantics permit us to do:
>
> if (dentry->d_flags & DCACHE_REVAL_FORCE_FAIL) {
> spin_lock(&dentry->d_lock);
> if (dentry->d_flags & DCACHE_REVAL_FORCE_FAIL) {
> dentry->d_flags &= ~DCACHE_REVAL_FORCE_FAIL;
> spin_unlock(&dentry->d_lock);
> dput(dentry);
> return -ENOENT;
> }
> spin_unlock(&dentry->d_lock);
> }
>
> to avoid taking the lock sometimes?
Should be infrequently.
I always balk at leaving things outside a lock but we test again so that
should be fine. Will do.
>
> And we should have an unlikely() in there to tell the compiler to put the
> code somewhere less likely to chew up CPU cache.
Yep.
>
>
> Also, did you consider broadening the ->d_revalidate() semantics? It
> appears that all implementations return 0 or 1. You could teach the VFS to
> also recognise and act upon a -ve return value, and do this trickery within
> the autofs d_revalidate(), perhaps?
I did but decided against it because that would change a long standing
behaviour. Someone would surely get caught.
I could scan the tree and make the approiate changes.
Anyone else care to offer an opinion?
Ian
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