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proc: tweak comments about 2 stage open and everything 

Some comments were obsoleted since commit 05c0ae21c0 ("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>
This commit is contained in:
Alexey Dobriyan 2016-12-12 16:45:22 -08:00 committed by Linus Torvalds
parent 39a10ac23c
commit 492b2da605
1 changed files with 21 additions and 8 deletions
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29
fs/proc/inode.c
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@ -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.
*
* 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.
* 2) rmmod isn't blocked by opening file in /proc and sitting on
* the descriptor (including "rmmod foo </proc/foo" scenario).
*
* 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);