From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: linux-kernel@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
stable@vger.kernel.org, Gratian Crisan <gratian.crisan@ni.com>,
Mike Galbraith <efault@gmx.de>,
Thomas Gleixner <tglx@linutronix.de>,
Zhen Lei <thunder.leizhen@huawei.com>,
Joe Korty <joe.korty@concurrent-rt.com>
Subject: [PATCH 4.4 08/11] futex: Handle transient "ownerless" rtmutex state correctly
Date: Sun, 8 Aug 2021 09:22:43 +0200 [thread overview]
Message-ID: <20210808072217.606471864@linuxfoundation.org> (raw)
In-Reply-To: <20210808072217.322468704@linuxfoundation.org>
From: Mike Galbraith <efault@gmx.de>
[ Upstream commit 9f5d1c336a10c0d24e83e40b4c1b9539f7dba627 ]
Gratian managed to trigger the BUG_ON(!newowner) in fixup_pi_state_owner().
This is one possible chain of events leading to this:
Task Prio Operation
T1 120 lock(F)
T2 120 lock(F) -> blocks (top waiter)
T3 50 (RT) lock(F) -> boosts T1 and blocks (new top waiter)
XX timeout/ -> wakes T2
signal
T1 50 unlock(F) -> wakes T3 (rtmutex->owner == NULL, waiter bit is set)
T2 120 cleanup -> try_to_take_mutex() fails because T3 is the top waiter
and the lower priority T2 cannot steal the lock.
-> fixup_pi_state_owner() sees newowner == NULL -> BUG_ON()
The comment states that this is invalid and rt_mutex_real_owner() must
return a non NULL owner when the trylock failed, but in case of a queued
and woken up waiter rt_mutex_real_owner() == NULL is a valid transient
state. The higher priority waiter has simply not yet managed to take over
the rtmutex.
The BUG_ON() is therefore wrong and this is just another retry condition in
fixup_pi_state_owner().
Drop the locks, so that T3 can make progress, and then try the fixup again.
Gratian provided a great analysis, traces and a reproducer. The analysis is
to the point, but it confused the hell out of that tglx dude who had to
page in all the futex horrors again. Condensed version is above.
[ tglx: Wrote comment and changelog ]
Fixes: c1e2f0eaf015 ("futex: Avoid violating the 10th rule of futex")
Reported-by: Gratian Crisan <gratian.crisan@ni.com>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/87a6w6x7bb.fsf@ni.com
Link: https://lore.kernel.org/r/87sg9pkvf7.fsf@nanos.tec.linutronix.de
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Acked-by: Joe Korty <joe.korty@concurrent-rt.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/futex.c | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2437,10 +2437,22 @@ retry:
}
/*
- * Since we just failed the trylock; there must be an owner.
+ * The trylock just failed, so either there is an owner or
+ * there is a higher priority waiter than this one.
*/
newowner = rt_mutex_owner(&pi_state->pi_mutex);
- BUG_ON(!newowner);
+ /*
+ * If the higher priority waiter has not yet taken over the
+ * rtmutex then newowner is NULL. We can't return here with
+ * that state because it's inconsistent vs. the user space
+ * state. So drop the locks and try again. It's a valid
+ * situation and not any different from the other retry
+ * conditions.
+ */
+ if (unlikely(!newowner)) {
+ err = -EAGAIN;
+ goto handle_fault;
+ }
} else {
WARN_ON_ONCE(argowner != current);
if (oldowner == current) {
next prev parent reply other threads:[~2021-08-08 7:24 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-08 7:22 [PATCH 4.4 00/11] 4.4.280-rc1 review Greg Kroah-Hartman
2021-08-08 7:22 ` [PATCH 4.4 01/11] futex: Rename free_pi_state() to put_pi_state() Greg Kroah-Hartman
2021-08-08 7:22 ` [PATCH 4.4 02/11] futex: Cleanup refcounting Greg Kroah-Hartman
2021-08-08 7:22 ` [PATCH 4.4 03/11] futex,rt_mutex: Introduce rt_mutex_init_waiter() Greg Kroah-Hartman
2021-08-08 7:22 ` [PATCH 4.4 04/11] futex: Pull rt_mutex_futex_unlock() out from under hb->lock Greg Kroah-Hartman
2021-08-08 7:22 ` [PATCH 4.4 05/11] futex: Rework futex_lock_pi() to use rt_mutex_*_proxy_lock() Greg Kroah-Hartman
2021-08-08 7:22 ` [PATCH 4.4 06/11] futex: Futex_unlock_pi() determinism Greg Kroah-Hartman
2021-08-08 7:22 ` [PATCH 4.4 07/11] rtmutex: Make wait_lock irq safe Greg Kroah-Hartman
2021-08-08 7:22 ` Greg Kroah-Hartman [this message]
2021-08-08 7:22 ` [PATCH 4.4 09/11] futex: Avoid freeing an active timer Greg Kroah-Hartman
2021-08-08 7:22 ` [PATCH 4.4 10/11] futex,rt_mutex: Fix rt_mutex_cleanup_proxy_lock() Greg Kroah-Hartman
2021-08-08 7:22 ` [PATCH 4.4 11/11] rcu: Update documentation of rcu_read_unlock() Greg Kroah-Hartman
2021-08-08 16:00 ` [PATCH 4.4 00/11] 4.4.280-rc1 review Guenter Roeck
2021-08-10 15:44 ` Greg Kroah-Hartman
2021-08-09 14:14 ` Naresh Kamboju
-- strict thread matches above, loose matches on Subject: below --
2021-08-02 13:46 [PATCH 4.4 00/11] Fix a potential infinite loop in RT futex-pi scenarios Zhen Lei
2021-08-02 13:46 ` [PATCH 4.4 08/11] futex: Handle transient "ownerless" rtmutex state correctly Zhen Lei
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20210808072217.606471864@linuxfoundation.org \
--to=gregkh@linuxfoundation.org \
--cc=efault@gmx.de \
--cc=gratian.crisan@ni.com \
--cc=joe.korty@concurrent-rt.com \
--cc=linux-kernel@vger.kernel.org \
--cc=stable@vger.kernel.org \
--cc=tglx@linutronix.de \
--cc=thunder.leizhen@huawei.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).