Christian Brauner brauner@kernel.org writes:

On Tue, May 30, 2023 at 06:57:48PM -0700, Eric Biggers wrote:

...

On Tue, May 30, 2023 at 11:32:28AM -0700, Benjamin Segall wrote:

...

autoremove_wake_function uses list_del_init_careful, so should epoll's more aggressive variant. It only doesn't because it was copied from an older wait.c rather than the most recent.

Fixes: a16ceb139610 ("epoll: autoremove wakers even more aggressively") Signed-off-by: Ben Segall bsegall@google.com Cc: stable@vger.kernel.org

---

fs/eventpoll.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c index 52954d4637b5..081df056398a 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -1756,11 +1756,11 @@ static struct timespec64 *ep_timeout_to_timespec(struct timespec64 *to, long ms) static int ep_autoremove_wake_function(struct wait_queue_entry *wq_entry, unsigned int mode, int sync, void *key) { int ret = default_wake_function(wq_entry, mode, sync, key);

• list_del_init(&wq_entry->entry);

• list_del_init_careful(&wq_entry->entry); return ret;

}

Can you please provide a more detailed explanation about why list_del_init_careful() is needed here?

Yeah, this needs more explanation... Next time someone looks at this code and there's a *_careful() added they'll want to know why.

So the general reason is the same as with autoremove_wake_function, it pairs with the list_entry_careful in ep_poll (which is epoll's modified copy of finish_wait).

I think the original actual _problem_ was a -stable issue that was fixed instead by doing additional backports, so this may just avoid potential extra loops and avoid potential compiler shenanigans from the data race.

autoremove_wake_function uses list_del_init_careful, so should epoll's more aggressive variant. It only doesn't because it was copied from an older wait.c rather than the most recent.

Fixes: a16ceb139610 ("epoll: autoremove wakers even more aggressively") Signed-off-by: Ben Segall bsegall@google.com Cc: stable@vger.kernel.org Change-Id: Icca05359250297f091779c9dcf4fefea92ee8c93 --- fs/eventpoll.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c index 980483455cc09..266d45c7685b4 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -1803,11 +1803,15 @@ static struct timespec64 *ep_timeout_to_timespec(struct timespec64 *to, long ms) static int ep_autoremove_wake_function(struct wait_queue_entry *wq_entry, unsigned int mode, int sync, void *key) { int ret = default_wake_function(wq_entry, mode, sync, key);

- list_del_init(&wq_entry->entry); + /* + * Pairs with list_empty_careful in ep_poll, and ensures future loop + * iterations see the cause of this wakeup. + */ + list_del_init_careful(&wq_entry->entry); return ret; }

/** * ep_poll - Retrieves ready events, and delivers them to the caller-supplied