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[79.37.206.118]) by smtp.gmail.com with ESMTPSA id dd24sm944567edb.45.2021.07.14.05.20.30 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 14 Jul 2021 05:20:30 -0700 (PDT) Subject: Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal() To: Paolo Bonzini , Jason Wang , Thomas Gleixner , Sebastian Andrzej Siewior , "Michael S. Tsirkin" , Juri Lelli Cc: LKML , Al Viro , He Zhe References: <8dfc0ee9-b97a-8ca8-d057-31c8cad3f5b6@redhat.com> <475f84e2-78ee-1a24-ef57-b16c1f2651ed@redhat.com> From: Daniel Bristot de Oliveira Message-ID: Date: Wed, 14 Jul 2021 14:20:29 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <475f84e2-78ee-1a24-ef57-b16c1f2651ed@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > 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 > Fixes: b5e683d5cab8 ("eventfd: track eventfd_signal() recursion depth") > Cc: stable@vger.kernel.org > Cc: He Zhe > Cc: Jens Axboe > Signed-off-by: Paolo Bonzini > > 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 >  #include >  #include > +#include >  #include >  #include >  #include > @@ -25,6 +26,7 @@ >  #include >  #include >   > +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; >  } >