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Message-ID: <ZLAK+FA3qgbHW0YK@google.com>
Date: Thu, 13 Jul 2023 07:33:05 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Christian Brauner <brauner@...nel.org>
Cc: linux-fsdevel@...r.kernel.org,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Paolo Bonzini <pbonzini@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
David Woodhouse <dwmw2@...radead.org>,
Paul Durrant <paul@....org>, Oded Gabbay <ogabbay@...nel.org>,
Wu Hao <hao.wu@...el.com>, Tom Rix <trix@...hat.com>,
Moritz Fischer <mdf@...nel.org>, Xu Yilun <yilun.xu@...el.com>,
Zhenyu Wang <zhenyuw@...ux.intel.com>,
Zhi Wang <zhi.a.wang@...el.com>,
Jani Nikula <jani.nikula@...ux.intel.com>,
Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
Rodrigo Vivi <rodrigo.vivi@...el.com>,
Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
David Airlie <airlied@...il.com>,
Daniel Vetter <daniel@...ll.ch>,
Leon Romanovsky <leon@...nel.org>,
Jason Gunthorpe <jgg@...pe.ca>,
Frederic Barrat <fbarrat@...ux.ibm.com>,
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Arnd Bergmann <arnd@...db.de>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Eric Farman <farman@...ux.ibm.com>,
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Johannes Weiner <hannes@...xchg.org>,
Michal Hocko <mhocko@...nel.org>,
Roman Gushchin <roman.gushchin@...ux.dev>,
Shakeel Butt <shakeelb@...gle.com>,
Muchun Song <muchun.song@...ux.dev>,
Kirti Wankhede <kwankhede@...dia.com>, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
linux-fpga@...r.kernel.org, intel-gvt-dev@...ts.freedesktop.org,
intel-gfx@...ts.freedesktop.org, linux-rdma@...r.kernel.org,
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linux-aio@...ck.org, cgroups@...r.kernel.org, linux-mm@...ck.org,
Jens Axboe <axboe@...nel.dk>,
Pavel Begunkov <asml.silence@...il.com>,
io-uring@...r.kernel.org
Subject: Re: [PATCH 2/2] eventfd: simplify eventfd_signal_mask()
On Thu, Jul 13, 2023, Christian Brauner wrote:
> diff --git a/fs/eventfd.c b/fs/eventfd.c
> index dc9e01053235..077be5da72bd 100644
> --- a/fs/eventfd.c
> +++ b/fs/eventfd.c
> @@ -43,9 +43,10 @@ struct eventfd_ctx {
> int id;
> };
>
> -__u64 eventfd_signal_mask(struct eventfd_ctx *ctx, __u64 n, __poll_t mask)
> +bool eventfd_signal_mask(struct eventfd_ctx *ctx, __poll_t mask)
> {
> unsigned long flags;
> + __u64 n = 1;
>
> /*
> * Deadlock or stack overflow issues can happen if we recurse here
> @@ -68,7 +69,7 @@ __u64 eventfd_signal_mask(struct eventfd_ctx *ctx, __u64 n, __poll_t mask)
> current->in_eventfd = 0;
> spin_unlock_irqrestore(&ctx->wqh.lock, flags);
>
> - return n;
> + return n == 1;
> }
...
> @@ -58,13 +58,12 @@ static inline struct eventfd_ctx *eventfd_ctx_fdget(int fd)
> return ERR_PTR(-ENOSYS);
> }
>
> -static inline int eventfd_signal(struct eventfd_ctx *ctx)
> +static inline bool eventfd_signal(struct eventfd_ctx *ctx)
> {
> return -ENOSYS;
> }
>
> -static inline int eventfd_signal_mask(struct eventfd_ctx *ctx, __u64 n,
> - unsigned mask)
> +static inline bool eventfd_signal_mask(struct eventfd_ctx *ctx, unsigned mask)
> {
> return -ENOSYS;
This will morph to "true" for what should be an error case. One option would be
to have eventfd_signal_mask() return 0/-errno instead of the count, but looking
at all the callers, nothing ever actually consumes the result.
KVMGT morphs failure into -EFAULT
if (vgpu->msi_trigger && eventfd_signal(vgpu->msi_trigger, 1) != 1)
return -EFAULT;
but the only caller of that user ignores the return value.
if (vgpu_vreg(vgpu, i915_mmio_reg_offset(GEN8_MASTER_IRQ))
& ~GEN8_MASTER_IRQ_CONTROL)
inject_virtual_interrupt(vgpu);
The sample driver in samples/vfio-mdev/mtty.c uses a similar pattern: prints an
error but otherwise ignores the result.
So why not return nothing? That will simplify eventfd_signal_mask() a wee bit
more, and eliminate that bizarre return value confusion for the ugly stubs, e.g.
void eventfd_signal_mask(struct eventfd_ctx *ctx, unsigned mask)
{
unsigned long flags;
/*
* Deadlock or stack overflow issues can happen if we recurse here
* through waitqueue wakeup handlers. If the caller users potentially
* nested waitqueues with custom wakeup handlers, then it should
* check eventfd_signal_allowed() before calling this function. If
* it returns false, the eventfd_signal() call should be deferred to a
* safe context.
*/
if (WARN_ON_ONCE(current->in_eventfd))
return;
spin_lock_irqsave(&ctx->wqh.lock, flags);
current->in_eventfd = 1;
if (ctx->count < ULLONG_MAX)
ctx->count++;
if (waitqueue_active(&ctx->wqh))
wake_up_locked_poll(&ctx->wqh, EPOLLIN | mask);
current->in_eventfd = 0;
spin_unlock_irqrestore(&ctx->wqh.lock, flags);
}
You could even go further and unify the real and stub versions of eventfd_signal().
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