__d_lookup_done() wakes waiters on dentry->d_wait. On PREEMPT_RT we are
not allowed to do that with preemption disabled, since the wakeup
acquired wait_queue_head::lock, which is a "sleeping" spinlock on RT.
Calling it under dentry->d_lock is not a problem, since that is also a
"sleeping" spinlock on the same configs. Unfortunately, two of its
callers (__d_add() and __d_move()) are holding more than just ->d_lock
and that needs to be dealt with.
The key observation is that wakeup can be moved to any point before
dropping ->d_lock.
As a first step to solve this, move the wake up outside of the
hlist_bl_lock() held section.
This is safe because:
Waiters get inserted into ->d_wait only after they'd taken ->d_lock
and observed DCACHE_PAR_LOOKUP in flags. As long as they are
woken up (and evicted from the queue) between the moment __d_lookup_done()
has removed DCACHE_PAR_LOOKUP and dropping ->d_lock, we are safe,
since the waitqueue ->d_wait points to won't get destroyed without
having __d_lookup_done(dentry) called (under ->d_lock).
->d_wait is set only by d_alloc_parallel() and only in case when
it returns a freshly allocated in-lookup dentry. Whenever that happens,
we are guaranteed that __d_lookup_done() will be called for resulting
dentry (under ->d_lock) before the wq in question gets destroyed.
With two exceptions wq lives in call frame of the caller of
d_alloc_parallel() and we have an explicit d_lookup_done() on the
resulting in-lookup dentry before we leave that frame.
One of those exceptions is nfs_call_unlink(), where wq is embedded into
(dynamically allocated) struct nfs_unlinkdata. It is destroyed in
nfs_async_unlink_release() after an explicit d_lookup_done() on the
dentry wq went into.
Remaining exception is d_add_ci(). There wq is what we'd found in
->d_wait of d_add_ci() argument. Callers of d_add_ci() are two
instances of ->d_lookup() and they must have been given an in-lookup
dentry. Which means that they'd been called by __lookup_slow() or
lookup_open(), with wq in the call frame of one of those.
Result of d_alloc_parallel() in d_add_ci() is fed to
d_splice_alias(), which either returns non-NULL (and d_add_ci() does
d_lookup_done()) or feeds dentry to __d_add() that will do
__d_lookup_done() under ->d_lock. That concludes the analysis.
Let __d_lookup_unhash():
1) Lock the lookup hash and clear DCACHE_PAR_LOOKUP
2) Unhash the dentry
3) Retrieve and clear dentry::d_wait
4) Unlock the hash and return the retrieved waitqueue head pointer
5) Let the caller handle the wake up.
6) Rename __d_lookup_done() to __d_lookup_unhash_wake() to enforce
build failures for OOT code that used __d_lookup_done() and is not
aware of the new return value.
This does not yet solve the PREEMPT_RT problem completely because
preemption is still disabled due to i_dir_seq being held for write. This
will be addressed in subsequent steps.
An alternative solution would be to switch the waitqueue to a simple
waitqueue, but aside of Linus not being a fan of them, moving the wake up
closer to the place where dentry::lock is unlocked reduces lock contention
time for the woken up waiter.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20220613140712.77932-3-bigeasy@xxxxxxxxxxxxx
---
fs/dcache.c | 35 ++++++++++++++++++++++++++++-------
include/linux/dcache.h | 9 +++------
2 files changed, 31 insertions(+), 13 deletions(-)
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2712,32 +2712,51 @@ struct dentry *d_alloc_parallel(struct d
}
EXPORT_SYMBOL(d_alloc_parallel);
-void __d_lookup_done(struct dentry *dentry)
+/*
+ * - Unhash the dentry
+ * - Retrieve and clear the waitqueue head in dentry
+ * - Return the waitqueue head
+ */
+static wait_queue_head_t *__d_lookup_unhash(struct dentry *dentry)
{
- struct hlist_bl_head *b = in_lookup_hash(dentry->d_parent,
- dentry->d_name.hash);
+ wait_queue_head_t *d_wait;
+ struct hlist_bl_head *b;
+
+ lockdep_assert_held(&dentry->d_lock);
+
+ b = in_lookup_hash(dentry->d_parent, dentry->d_name.hash);
hlist_bl_lock(b);
dentry->d_flags &= ~DCACHE_PAR_LOOKUP;
__hlist_bl_del(&dentry->d_u.d_in_lookup_hash);
- wake_up_all(dentry->d_wait);
+ d_wait = dentry->d_wait;
dentry->d_wait = NULL;
hlist_bl_unlock(b);
INIT_HLIST_NODE(&dentry->d_u.d_alias);
INIT_LIST_HEAD(&dentry->d_lru);
+ return d_wait;
+}
+
+void __d_lookup_unhash_wake(struct dentry *dentry)
+{
+ spin_lock(&dentry->d_lock);
+ wake_up_all(__d_lookup_unhash(dentry));
+ spin_unlock(&dentry->d_lock);
}
-EXPORT_SYMBOL(__d_lookup_done);
+EXPORT_SYMBOL(__d_lookup_unhash_wake);
/* inode->i_lock held if inode is non-NULL */
static inline void __d_add(struct dentry *dentry, struct inode *inode)
{
+ wait_queue_head_t *d_wait;
struct inode *dir = NULL;
unsigned n;
spin_lock(&dentry->d_lock);
if (unlikely(d_in_lookup(dentry))) {
dir = dentry->d_parent->d_inode;
n = start_dir_add(dir);
- __d_lookup_done(dentry);
+ d_wait = __d_lookup_unhash(dentry);
+ wake_up_all(d_wait);
}
if (inode) {
unsigned add_flags = d_flags_for_inode(inode);
@@ -2896,6 +2915,7 @@ static void __d_move(struct dentry *dent
bool exchange)
{
struct dentry *old_parent, *p;
+ wait_queue_head_t *d_wait;
struct inode *dir = NULL;
unsigned n;
@@ -2926,7 +2946,8 @@ static void __d_move(struct dentry *dent
if (unlikely(d_in_lookup(target))) {
dir = target->d_parent->d_inode;
n = start_dir_add(dir);
- __d_lookup_done(target);
+ d_wait = __d_lookup_unhash(target);
+ wake_up_all(d_wait);
}
write_seqcount_begin(&dentry->d_seq);
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -349,7 +349,7 @@ static inline void dont_mount(struct den
spin_unlock(&dentry->d_lock);
}
-extern void __d_lookup_done(struct dentry *);
+extern void __d_lookup_unhash_wake(struct dentry *dentry);
static inline int d_in_lookup(const struct dentry *dentry)
{
@@ -358,11 +358,8 @@ static inline int d_in_lookup(const stru
static inline void d_lookup_done(struct dentry *dentry)
{
- if (unlikely(d_in_lookup(dentry))) {
- spin_lock(&dentry->d_lock);
- __d_lookup_done(dentry);
- spin_unlock(&dentry->d_lock);
- }
+ if (unlikely(d_in_lookup(dentry)))
+ __d_lookup_unhash_wake(dentry);
}
extern void dput(struct dentry *);
