Messages in this thread |  | | Subject | Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal() | From | Daniel Bristot de Oliveira <> | Date | Wed, 14 Jul 2021 14:20:29 +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!!):
Makes sense, testing the patch.
-- Daniel
> --------------- 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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