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[79.37.206.118]) by smtp.gmail.com with ESMTPSA id q24sm2035993edc.82.2021.07.15.01.22.46 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 15 Jul 2021 01:22:47 -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: Thu, 15 Jul 2021 10:22:46 +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!!): the lock needs to be per-pcu... but so far, so good. I will continue using the system in the next days to see if it blows on another way. The patch looks like this now: ------------------------- 8< --------------------- 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 Suggested-by: Paolo Bonzini Fixes: b5e683d5cab8 ("eventfd: track eventfd_signal() recursion depth") Cc: stable@vger.kernel.org Cc: He Zhe Cc: Jens Axboe Co-developed-by: Paolo Bonzini Signed-off-by: Paolo Bonzini Co-developed-by: Daniel Bristot de Oliveira Signed-off-by: Daniel Bristot de Oliveira --- fs/eventfd.c | 27 ++++++++++++++++++++++----- include/linux/eventfd.h | 7 +------ 2 files changed, 23 insertions(+), 11 deletions(-) diff --git a/fs/eventfd.c b/fs/eventfd.c index e265b6dd4f34..9754fcd38690 100644 --- a/fs/eventfd.c +++ b/fs/eventfd.c @@ -12,6 +12,7 @@ #include #include #include +#include #include #include #include @@ -25,8 +26,6 @@ #include #include -DEFINE_PER_CPU(int, eventfd_wake_count); - static DEFINE_IDA(eventfd_ida); struct eventfd_ctx { @@ -45,6 +44,20 @@ struct eventfd_ctx { int id; }; +struct event_fd_recursion { + local_lock_t lock; + int count; +}; + +static DEFINE_PER_CPU(struct event_fd_recursion, event_fd_recursion) = { + .lock = INIT_LOCAL_LOCK(lock), +}; + +bool eventfd_signal_count(void) +{ + return this_cpu_read(event_fd_recursion.count); +} + /** * eventfd_signal - Adds @n to the eventfd counter. * @ctx: [in] Pointer to the eventfd context. @@ -71,18 +84,22 @@ __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(&event_fd_recursion.lock); + if (WARN_ON_ONCE(this_cpu_read(event_fd_recursion.count))) { + local_unlock(&event_fd_recursion.lock); return 0; + } spin_lock_irqsave(&ctx->wqh.lock, flags); - this_cpu_inc(eventfd_wake_count); + this_cpu_inc(event_fd_recursion.count); if (ULLONG_MAX - ctx->count < n) n = ULLONG_MAX - ctx->count; ctx->count += n; if (waitqueue_active(&ctx->wqh)) wake_up_locked_poll(&ctx->wqh, EPOLLIN); - this_cpu_dec(eventfd_wake_count); + this_cpu_dec(event_fd_recursion.count); spin_unlock_irqrestore(&ctx->wqh.lock, flags); + local_unlock(&event_fd_recursion.lock); return n; } diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h index fa0a524baed0..ca89d6c409c1 100644 --- a/include/linux/eventfd.h +++ b/include/linux/eventfd.h @@ -43,12 +43,7 @@ int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_entry_t *w __u64 *cnt); void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt); -DECLARE_PER_CPU(int, eventfd_wake_count); - -static inline bool eventfd_signal_count(void) -{ - return this_cpu_read(eventfd_wake_count); -} +bool eventfd_signal_count(void); #else /* CONFIG_EVENTFD */ --