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SubjectRe: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()
Date
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;
}
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