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 | Thu, 15 Jul 2021 07:58:28 +0200 |
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On 15/07/21 06:14, Jason Wang wrote: >> 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. > > But local_lock only disable migration not preemption.
On mainline PREEMPT_RT, local_lock is an array of per-CPU spinlocks. When two eventfd_signals run on the same CPU and one is preempted, the spinlocks avoid that the second sees eventfd_wake_count > 0.
Thanks,
Paolo
> Or anything I missed here? > > Thanks > > >> >> 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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