[PATCH RESEND v2 04/23] proc: wait for the last user of an entry with wait_var_event()

[Christian Brauner]

proc_entry_rundown() waits for the last user of an entry with a
completion on its stack that it points the entry at while it waits.
That's a pointer in every struct proc_dir_entry for something that
happens once in the life of an entry, if at all.

Wait on the count itself with wait_var_event() instead and let
unuse_pde() wake the waiter with wake_up_var() when the count hits the
bias. The waiters sleep on the hashed wait queues behind that, so
procfs doesn't need a wait queue of its own for this, neither in the
entry nor a global one. kernfs keeps one per root for the same thing,
deactivate_waitq. Note that atomic_dec_return() is fully ordered, so
the count is visible before wake_up_var() looks for waiters.

No functional changes.

Signed-off-by: Christian Brauner (Amutable) <brauner@xxxxxxxxxx>
---
 fs/proc/inode.c    | 7 +++----
 fs/proc/internal.h | 1 -
 2 files changed, 3 insertions(+), 5 deletions(-)

diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index 66a3688757dd..8795add1898d 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -25,6 +25,7 @@
 #include <linux/slab.h>
 #include <linux/mount.h>
 #include <linux/bug.h>
+#include <linux/wait_bit.h>
 
 #include "internal.h"
 
@@ -203,7 +204,7 @@ static inline int use_pde(struct proc_dir_entry *pde)
 static void unuse_pde(struct proc_dir_entry *pde)
 {
 	if (unlikely(atomic_dec_return(&pde->in_use) == BIAS))
-		complete(pde->pde_unload_completion);
+		wake_up_var(&pde->in_use);
 }
 
 /* Pin an entry that can go away around a call into it, see use_pde(). */
@@ -266,11 +267,9 @@ static void close_pdeo(struct proc_dir_entry *pde, struct pde_opener *pdeo)
 
 void proc_entry_rundown(struct proc_dir_entry *de)
 {
-	DECLARE_COMPLETION_ONSTACK(c);
 	/* Wait until all existing callers into module are done. */
-	de->pde_unload_completion = &c;
 	if (atomic_add_return(BIAS, &de->in_use) != BIAS)
-		wait_for_completion(&c);
+		wait_var_event(&de->in_use, atomic_read(&de->in_use) == BIAS);
 
 	/* ->pde_openers list can't grow from now on. */
 
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index 6b61d09ac5e5..ab4d7bd1fb75 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -38,7 +38,6 @@ struct proc_dir_entry {
 	struct list_head pde_openers;	/* who did ->open, but not ->release */
 	/* protects ->pde_openers and all struct pde_opener instances */
 	spinlock_t pde_unload_lock;
-	struct completion *pde_unload_completion;
 	const struct inode_operations *proc_iops;
 	union {
 		const struct proc_ops *proc_ops;

-- 
2.53.0