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Call retain_dentry() with refcount 0

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Instead of bumping it from 0 to 1, calling retain_dentry(), then
decrementing it back to 0 (with ->d_lock held all the way through),
just leave refcount at 0 through all of that.

It will have a visible effect for ->d_delete() - now it can be
called with refcount 0 instead of 1 and it can no longer play
silly buggers with dropping/regaining ->d_lock.  Not that any
in-tree instances tried to (it's pretty hard to get right).

Any out-of-tree ones will have to adjust (assuming they need any
changes).

Note that we do not need to extend rcu-critical area here - we have
verified that refcount is non-negative after having grabbed ->d_lock,
so nobody will be able to free dentry until they get into __dentry_kill(),
which won't happen until they manage to grab ->d_lock.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2023-10-30 01:09:50 -04:00
parent b06c684d39
commit 2f42f1eb90
2 changed files with 10 additions and 8 deletions
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8
Documentation/filesystems/porting.rst
View File

@ -1070,3 +1070,11 @@ The list of children anchored in parent dentry got turned into hlist now.
Field names got changed (->d_children/->d_sib instead of ->d_subdirs/->d_child
for anchor/entries resp.), so any affected places will be immediately caught
by compiler.
---
**mandatory**
->d_delete() instances are now called for dentries with ->d_lock held
and refcount equal to 0. They are not permitted to drop/regain ->d_lock.
None of in-tree instances did anything of that sort. Make sure yours do not...

10
fs/dcache.c
View File

@ -888,15 +888,14 @@ void dput(struct dentry *dentry)
}
/* Slow case: now with the dentry lock held */
dentry->d_lockref.count = 1;
rcu_read_unlock();
if (likely(retain_dentry(dentry))) {
dentry->d_lockref.count--;
spin_unlock(&dentry->d_lock);
return;
}
dentry->d_lockref.count = 1;
dentry = dentry_kill(dentry);
}
}
@ -921,13 +920,8 @@ void dput_to_list(struct dentry *dentry, struct list_head *list)
return;
}
rcu_read_unlock();
dentry->d_lockref.count = 1;
if (!retain_dentry(dentry)) {
--dentry->d_lockref.count;
if (!retain_dentry(dentry))
to_shrink_list(dentry, list);
} else {
--dentry->d_lockref.count;
}
spin_unlock(&dentry->d_lock);
}