From: Paolo Bonzini <pbonzini@redhat.com>
To: Jason Wang <jasowang@redhat.com>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
"Michael S. Tsirkin" <mst@redhat.com>,
Juri Lelli <jlelli@redhat.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
Al Viro <viro@zeniv.linux.org.uk>, He Zhe <zhe.he@windriver.com>
Subject: Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()
Date: Wed, 14 Jul 2021 12:35:27 +0200 [thread overview]
Message-ID: <475f84e2-78ee-1a24-ef57-b16c1f2651ed@redhat.com> (raw)
In-Reply-To: <f0254740-944d-201b-9a66-9db1fe480ca6@redhat.com>
On 14/07/21 11:23, Jason Wang wrote:
>> This was added in 2020, so it's unlikely to be the direct cause of the
>> change. What is a known-good version for the host?
>>
>> Since it is not KVM stuff, I'm CCing Michael and Jason.
>
> I think this can be probably fixed here:
>
> https://lore.kernel.org/lkml/20210618084412.18257-1-zhe.he@windriver.com/
That seems wrong; in particular it wouldn't protect against AB/BA deadlocks.
In fact, the bug is with the locking; the code assumes that
spin_lock_irqsave/spin_unlock_irqrestore is non-preemptable and therefore
increments and decrements the percpu variable inside the critical section.
This obviously does not fly with PREEMPT_RT; the right fix should be
using a local_lock. Something like this (untested!!):
--------------- 8< ---------------
From: Paolo Bonzini <pbonzini@redhat.com>
Subject: [PATCH] eventfd: protect eventfd_wake_count with a local_lock
eventfd_signal assumes that spin_lock_irqsave/spin_unlock_irqrestore is
non-preemptable and therefore increments and decrements the percpu
variable inside the critical section.
This obviously does not fly with PREEMPT_RT. If eventfd_signal is
preempted and an unrelated thread calls eventfd_signal, the result is
a spurious WARN. To avoid this, protect the percpu variable with a
local_lock.
Reported-by: Daniel Bristot de Oliveira <bristot@redhat.com>
Fixes: b5e683d5cab8 ("eventfd: track eventfd_signal() recursion depth")
Cc: stable@vger.kernel.org
Cc: He Zhe <zhe.he@windriver.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/fs/eventfd.c b/fs/eventfd.c
index e265b6dd4f34..7d27b6e080ea 100644
--- a/fs/eventfd.c
+++ b/fs/eventfd.c
@@ -12,6 +12,7 @@
#include <linux/fs.h>
#include <linux/sched/signal.h>
#include <linux/kernel.h>
+#include <linux/local_lock.h>
#include <linux/slab.h>
#include <linux/list.h>
#include <linux/spinlock.h>
@@ -25,6 +26,7 @@
#include <linux/idr.h>
#include <linux/uio.h>
+static local_lock_t eventfd_wake_lock = INIT_LOCAL_LOCK(eventfd_wake_lock);
DEFINE_PER_CPU(int, eventfd_wake_count);
static DEFINE_IDA(eventfd_ida);
@@ -71,8 +73,11 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
* it returns true, the eventfd_signal() call should be deferred to a
* safe context.
*/
- if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count)))
+ local_lock(&eventfd_wake_lock);
+ if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count))) {
+ local_unlock(&eventfd_wake_lock);
return 0;
+ }
spin_lock_irqsave(&ctx->wqh.lock, flags);
this_cpu_inc(eventfd_wake_count);
@@ -83,6 +88,7 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
wake_up_locked_poll(&ctx->wqh, EPOLLIN);
this_cpu_dec(eventfd_wake_count);
spin_unlock_irqrestore(&ctx->wqh.lock, flags);
+ local_unlock(&eventfd_wake_lock);
return n;
}
next prev parent reply other threads:[~2021-07-14 10:35 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-14 8:01 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal() Daniel Bristot de Oliveira
2021-07-14 8:10 ` Paolo Bonzini
2021-07-14 9:23 ` Jason Wang
2021-07-14 10:35 ` Paolo Bonzini [this message]
2021-07-14 10:41 ` Michael S. Tsirkin
2021-07-14 10:44 ` Paolo Bonzini
2021-07-14 12:20 ` Daniel Bristot de Oliveira
2021-07-15 4:14 ` Jason Wang
2021-07-15 5:58 ` Paolo Bonzini
2021-07-15 6:45 ` Jason Wang
2021-07-15 8:22 ` Daniel Bristot de Oliveira
2021-07-15 8:44 ` He Zhe
2021-07-15 9:51 ` Paolo Bonzini
2021-07-15 10:10 ` He Zhe
2021-07-15 11:05 ` Paolo Bonzini
2021-07-16 2:26 ` Jason Wang
2021-07-16 2:43 ` He Zhe
2021-07-16 2:46 ` Jason Wang
2021-07-15 9:46 ` Paolo Bonzini
2021-07-15 12:34 ` Daniel Bristot de Oliveira
[not found] ` <20210715102249.2205-1-hdanton@sina.com>
2021-07-15 12:31 ` Daniel Bristot de Oliveira
[not found] ` <20210716020611.2288-1-hdanton@sina.com>
2021-07-16 6:54 ` Paolo Bonzini
[not found] ` <20210716075539.2376-1-hdanton@sina.com>
2021-07-16 7:59 ` Paolo Bonzini
[not found] ` <20210716093725.2438-1-hdanton@sina.com>
2021-07-16 11:55 ` Paolo Bonzini
[not found] ` <20210718124219.1521-1-hdanton@sina.com>
2021-07-19 15:38 ` Paolo Bonzini
[not found] ` <20210721070452.1008-1-hdanton@sina.com>
2021-07-21 7:25 ` Thomas Gleixner
[not found] ` <20210721101119.1103-1-hdanton@sina.com>
2021-07-21 10:59 ` Paolo Bonzini
[not found] ` <20210723022356.1301-1-hdanton@sina.com>
2021-07-23 7:59 ` Paolo Bonzini
[not found] ` <20210723094830.1375-1-hdanton@sina.com>
2021-07-23 10:56 ` Paolo Bonzini
[not found] ` <20210724043320.1654-1-hdanton@sina.com>
2021-07-26 11:03 ` Paolo Bonzini
2021-07-28 8:06 ` Thomas Gleixner
2021-07-28 10:21 ` Paolo Bonzini
2021-07-28 19:07 ` Thomas Gleixner
2021-07-29 11:01 ` [PATCH] eventfd: Make signal recursion protection a task bit Thomas Gleixner
2021-07-29 14:32 ` Daniel Bristot de Oliveira
2021-07-29 19:23 ` Daniel Bristot de Oliveira
2021-08-26 7:03 ` Jason Wang
2021-08-27 23:41 ` [tip: sched/core] " tip-bot2 for Thomas Gleixner
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