[RFC PATCH 04/27] untangling ep_call_nested(): it's all serialized on epmutex.

Al Viro
2020-10-04T02:39:42Z

IOW,
* no locking is needed to protect the list
* the list is actually a stack
* no need to check ->ctx
* it can bloody well be a static 5-element array - nobody is
going to be accessing it in parallel.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
fs/eventpoll.c | 80 ++++++++--------------------------------------------------
1 file changed, 11 insertions(+), 69 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index ef73d71a5dc8..43aecae0935c 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -109,25 +109,6 @@ struct epoll_filefd {
int fd;
} __packed;

-/*
- * Structure used to track possible nested calls, for too deep recursions
- * and loop cycles.
- */
-struct nested_call_node {
- struct list_head llink;
- void *cookie;
- void *ctx;
-};
-
-/*
- * This structure is used as collector for nested calls, to check for
- * maximum recursion dept and loop cycles.
- */
-struct nested_calls {
- struct list_head tasks_call_list;
- spinlock_t lock;
-};
-
/* Wait structure used by the poll hooks */
struct eppoll_entry {
/* List header used to link this structure to the "struct epitem" */
@@ -273,7 +254,8 @@ static DEFINE_MUTEX(epmutex);
static u64 loop_check_gen = 0;

/* Used to check for epoll file descriptor inclusion loops */
-static struct nested_calls poll_loop_ncalls;
+static void *cookies[EP_MAX_NESTS + 1];
+static int nesting;

/* Slab cache used to allocate "struct epitem" */
static struct kmem_cache *epi_cache __read_mostly;
@@ -348,13 +330,6 @@ static inline struct epitem *ep_item_from_wait(wait_queue_entry_t *p)
return container_of(p, struct eppoll_entry, wait)->base;
}

-/* Initialize the poll safe wake up structure */
-static void ep_nested_calls_init(struct nested_calls *ncalls)
-{
- INIT_LIST_HEAD(&ncalls->tasks_call_list);
- spin_lock_init(&ncalls->lock);
-}
-
/**
* ep_events_available - Checks if ready events might be available.
*
@@ -465,47 +440,20 @@ static inline void ep_set_busy_poll_napi_id(struct epitem *epi)
static int ep_call_nested(int (*nproc)(void *, void *, int), void *priv,
void *cookie)
{
- int error, call_nests = 0;
- unsigned long flags;
- struct nested_calls *ncalls = &poll_loop_ncalls;
- struct list_head *lsthead = &ncalls->tasks_call_list;
- struct nested_call_node *tncur;
- struct nested_call_node tnode;
+ int error, i;

- spin_lock_irqsave(&ncalls->lock, flags);
+ if (nesting > EP_MAX_NESTS) /* too deep nesting */
+ return -1;

- /*
- * Try to see if the current task is already inside this wakeup call.
- * We use a list here, since the population inside this set is always
- * very much limited.
- */
- list_for_each_entry(tncur, lsthead, llink) {
- if (tncur->ctx == current &&
- (tncur->cookie == cookie || ++call_nests > EP_MAX_NESTS)) {
- /*
- * Ops ... loop detected or maximum nest level reached.
- * We abort this wake by breaking the cycle itself.
- */
- error = -1;
- goto out_unlock;
- }
+ for (i = 0; i < nesting; i++) {
+ if (cookies[i] == cookie) /* loop detected */
+ return -1;
}
-
- /* Add the current task and cookie to the list */
- tnode.ctx = current;
- tnode.cookie = cookie;
- list_add(&tnode.llink, lsthead);
-
- spin_unlock_irqrestore(&ncalls->lock, flags);
+ cookies[nesting++] = cookie;

/* Call the nested function */
- error = (*nproc)(priv, cookie, call_nests);
-
- /* Remove the current task from the list */
- spin_lock_irqsave(&ncalls->lock, flags);
- list_del(&tnode.llink);
-out_unlock:
- spin_unlock_irqrestore(&ncalls->lock, flags);
+ error = (*nproc)(priv, cookie, nesting - 1);
+ nesting--;

return error;
}
@@ -2380,12 +2328,6 @@ static int __init eventpoll_init(void)
BUG_ON(max_user_watches < 0);

/*
- * Initialize the structure used to perform epoll file descriptor
- * inclusion loops checks.
- */
- ep_nested_calls_init(&poll_loop_ncalls);
-
- /*
* We can have many thousands of epitems, so prevent this from
* using an extra cache line on 64-bit (and smaller) CPUs
*/
--
2.11.0