Messages in this thread Patch in this message |  | | From | Paolo Bonzini <> | Subject | Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal() | Date | Wed, 14 Jul 2021 12:35:27 +0200 |
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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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