Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()

[Paolo Bonzini]

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@xxxxxxxxxxxxx/

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@xxxxxxxxxx>
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@xxxxxxxxxx>
Fixes: b5e683d5cab8 ("eventfd: track eventfd_signal() recursion depth")
Cc: stable@xxxxxxxxxxxxxxx
Cc: He Zhe <zhe.he@xxxxxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

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;
 }