proc: tweak comments about 2 stage open and everything

Some comments were obsoleted since commit 05c0ae21c034 ("try a saner
locking for pde_opener...").

Some new comments added.

Some confusing comments replaced with equally confusing ones.

Link: http://lkml.kernel.org/r/20161029160231.GD1246@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 21 insertions, 8 deletions

diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index 57f548e2eb59..783bc19644d1 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -138,6 +138,16 @@ static void unuse_pde(struct proc_dir_entry *pde)
 /* pde is locked */
 static void close_pdeo(struct proc_dir_entry *pde, struct pde_opener *pdeo)
 {
+        /*
+         * close() (proc_reg_release()) can't delete an entry and proceed:
+         * ->release hook needs to be available at the right moment.
+         *
+         * rmmod (remove_proc_entry() et al) can't delete an entry and proceed:
+         * "struct file" needs to be available at the right moment.
+         *
+         * Therefore, first process to enter this function does ->release() and
+         * signals its completion to the other process which does nothing.
+         */
         if (pdeo->closing) {
                 /* somebody else is doing that, just wait */
                 DECLARE_COMPLETION_ONSTACK(c);
@@ -152,6 +162,7 @@ static void close_pdeo(struct proc_dir_entry *pde, struct pde_opener *pdeo)
                 file = pdeo->file;
                 pde->proc_fops->release(file_inode(file), file);
                 spin_lock(&pde->pde_unload_lock);
+                /* After ->release. */
                 list_del(&pdeo->lh);
                 if (pdeo->c)
                         complete(pdeo->c);
@@ -167,6 +178,8 @@ void proc_entry_rundown(struct proc_dir_entry *de)
         if (atomic_add_return(BIAS, &de->in_use) != BIAS)
                 wait_for_completion(&c);
 
+        /* ->pde_openers list can't grow from now on. */
+
         spin_lock(&de->pde_unload_lock);
         while (!list_empty(&de->pde_openers)) {
                 struct pde_opener *pdeo;
@@ -312,14 +325,15 @@ static int proc_reg_open(struct inode *inode, struct file *file)
         struct pde_opener *pdeo;
 
         /*
-         * What for, you ask? Well, we can have open, rmmod, remove_proc_entry
-         * sequence. ->release won't be called because ->proc_fops will be
-         * cleared. Depending on complexity of ->release, consequences vary.
+         * Ensure that
+         * 1) PDE's ->release hook will be called no matter what
+         *    either normally by close()/->release, or forcefully by
+         *    rmmod/remove_proc_entry.
+         *
+         * 2) rmmod isn't blocked by opening file in /proc and sitting on
+         *    the descriptor (including "rmmod foo </proc/foo" scenario).
          *
-         * We can't wait for mercy when close will be done for real, it's
-         * deadlockable: rmmod foo </proc/foo . So, we're going to do ->release
-         * by hand in remove_proc_entry(). For this, save opener's credentials
-         * for later.
+         * Save every "struct file" with custom ->release hook.
          */
         pdeo = kmalloc(sizeof(struct pde_opener), GFP_KERNEL);
         if (!pdeo)
@@ -340,7 +354,6 @@ static int proc_reg_open(struct inode *inode, struct file *file)
                 pdeo->file = file;
                 pdeo->closing = false;
                 pdeo->c = NULL;
-                /* Strictly for "too late" ->release in proc_reg_release(). */
                 spin_lock(&pde->pde_unload_lock);
                 list_add(&pdeo->lh, &pde->pde_openers);
                 spin_unlock(&pde->pde_unload_lock);