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Subject: Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()
To:     Paolo Bonzini <pbonzini@redhat.com>,
        Jason Wang <jasowang@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>
References: <df278db6-1fc0-3d42-9c0e-f5a085c6351e@redhat.com>
 <8dfc0ee9-b97a-8ca8-d057-31c8cad3f5b6@redhat.com>
 <f0254740-944d-201b-9a66-9db1fe480ca6@redhat.com>
 <475f84e2-78ee-1a24-ef57-b16c1f2651ed@redhat.com>
From:   Daniel Bristot de Oliveira <bristot@redhat.com>
Message-ID: <a56ddd50-2cd1-f16e-5180-5232c449fbd0@redhat.com>
Date:   Thu, 15 Jul 2021 10:22:46 +0200
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On 7/14/21 12:35 PM, Paolo Bonzini wrote:
> 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!!):

the lock needs to be per-pcu... but so far, so good. I will continue using the
system in the next days to see if it blows on another way.

The patch looks like this now:

------------------------- 8< ---------------------
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>
Suggested-by: Paolo Bonzini <pbonzini@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>
Co-developed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Co-developed-by: Daniel Bristot de Oliveira <bristot@redhat.com>
Signed-off-by: Daniel Bristot de Oliveira <bristot@redhat.com>
---
 fs/eventfd.c            | 27 ++++++++++++++++++++++-----
 include/linux/eventfd.h |  7 +------
 2 files changed, 23 insertions(+), 11 deletions(-)

diff --git a/fs/eventfd.c b/fs/eventfd.c
index e265b6dd4f34..9754fcd38690 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>
@@ -25,8 +26,6 @@
 #include <linux/idr.h>
 #include <linux/uio.h>

-DEFINE_PER_CPU(int, eventfd_wake_count);
-
 static DEFINE_IDA(eventfd_ida);

 struct eventfd_ctx {
@@ -45,6 +44,20 @@ struct eventfd_ctx {
 	int id;
 };

+struct event_fd_recursion {
+	local_lock_t lock;
+	int count;
+};
+
+static DEFINE_PER_CPU(struct event_fd_recursion, event_fd_recursion) = {
+	.lock = INIT_LOCAL_LOCK(lock),
+};
+
+bool eventfd_signal_count(void)
+{
+	return this_cpu_read(event_fd_recursion.count);
+}
+
 /**
  * eventfd_signal - Adds @n to the eventfd counter.
  * @ctx: [in] Pointer to the eventfd context.
@@ -71,18 +84,22 @@ __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(&event_fd_recursion.lock);
+	if (WARN_ON_ONCE(this_cpu_read(event_fd_recursion.count))) {
+		local_unlock(&event_fd_recursion.lock);
 		return 0;
+	}

 	spin_lock_irqsave(&ctx->wqh.lock, flags);
-	this_cpu_inc(eventfd_wake_count);
+	this_cpu_inc(event_fd_recursion.count);
 	if (ULLONG_MAX - ctx->count < n)
 		n = ULLONG_MAX - ctx->count;
 	ctx->count += n;
 	if (waitqueue_active(&ctx->wqh))
 		wake_up_locked_poll(&ctx->wqh, EPOLLIN);
-	this_cpu_dec(eventfd_wake_count);
+	this_cpu_dec(event_fd_recursion.count);
 	spin_unlock_irqrestore(&ctx->wqh.lock, flags);
+	local_unlock(&event_fd_recursion.lock);

 	return n;
 }
diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h
index fa0a524baed0..ca89d6c409c1 100644
--- a/include/linux/eventfd.h
+++ b/include/linux/eventfd.h
@@ -43,12 +43,7 @@ int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx,
wait_queue_entry_t *w
 				  __u64 *cnt);
 void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt);

-DECLARE_PER_CPU(int, eventfd_wake_count);
-
-static inline bool eventfd_signal_count(void)
-{
-	return this_cpu_read(eventfd_wake_count);
-}
+bool eventfd_signal_count(void);

 #else /* CONFIG_EVENTFD */

--