From: Prateek Sood <prsood@codeaurora.org>
To: peterz@infradead.org, mingo@redhat.com, dbueso@suse.de,
prsood@codeaurora.org
Cc: linux-kernel@vger.kernel.org, sramana@codeaurora.org
Subject: [PATCH] percpu_rwsem: fix missed wakeup due to reordering of load
Date: Fri, 30 Nov 2018 20:40:56 +0530 [thread overview]
Message-ID: <1543590656-7157-1-git-send-email-prsood@codeaurora.org> (raw)
In a scenario where cpu_hotplug_lock percpu_rw_semaphore is already
acquired for read operation by P1 using percpu_down_read().
Now we have P1 in the path of releaseing the cpu_hotplug_lock and P2
is in the process of acquiring cpu_hotplug_lock.
P1 P2
percpu_up_read() path percpu_down_write() path
rcu_sync_enter() //gp_state=GP_PASSED
rcu_sync_is_idle() //returns false down_write(rw_sem)
__percpu_up_read()
[L] task = rcu_dereference(w->task) //NULL
smp_rmb() [S] w->task = current
smp_mb()
[L] readers_active_check() //fails
schedule()
[S] __this_cpu_dec(read_count)
Since load of task can result in NULL. This can lead to missed wakeup
in rcuwait_wake_up(). Above sequence violated the following constraint
in rcuwait_wake_up():
WAIT WAKE
[S] tsk = current [S] cond = true
MB (A) MB (B)
[L] cond [L] tsk
This can happen as smp_rmb() in rcuwait_wake_up() will provide ordering
of load before barrier with load and store after barrier for arm64
architecture. Here the requirement is to order store before smp_rmb()
with load after the smp_rmb().
For the usage of rcuwait_wake_up() in __percpu_up_read() full barrier
(smp_mb) is required to complete the constraint of rcuwait_wake_up().
Signed-off-by: Prateek Sood <prsood@codeaurora.org>
---
kernel/exit.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/exit.c b/kernel/exit.c
index f1d74f0..a10820d 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -306,7 +306,7 @@ void rcuwait_wake_up(struct rcuwait *w)
* MB (A) MB (B)
* [L] cond [L] tsk
*/
- smp_rmb(); /* (B) */
+ smp_mb(); /* (B) */
/*
* Avoid using task_rcu_dereference() magic as long as we are careful,
--
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.,
is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
next reply other threads:[~2018-11-30 15:11 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-30 15:10 Prateek Sood [this message]
2018-12-03 6:38 ` [PATCH] percpu_rwsem: fix missed wakeup due to reordering of load Davidlohr Bueso
2018-12-03 19:36 ` Prateek Sood
2018-12-12 14:26 ` Prateek Sood
2018-12-12 15:31 ` Davidlohr Bueso
2018-12-21 7:29 ` Prateek Sood
2018-12-21 9:45 ` Andrea Parri
2018-12-12 15:28 ` Andrea Parri
2018-12-21 6:35 ` Prateek Sood
2019-01-21 11:25 ` [tip:locking/core] sched/wait: Fix rcuwait_wake_up() ordering tip-bot for Prateek Sood
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=1543590656-7157-1-git-send-email-prsood@codeaurora.org \
--to=prsood@codeaurora.org \
--cc=dbueso@suse.de \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=peterz@infradead.org \
--cc=sramana@codeaurora.org \
/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).