linux-kernel - Re: [PATCH net v2] hvsock: fix epollout hang from race condition

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Date:   Tue, 18 Jun 2019 21:42:32 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     sunilmut@...rosoft.com
Cc:     kys@...rosoft.com, haiyangz@...rosoft.com, sthemmin@...rosoft.com,
        sashal@...nel.org, decui@...rosoft.com, mikelley@...rosoft.com,
        netdev@...r.kernel.org, linux-hyperv@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net v2] hvsock: fix epollout hang from race condition

From: Sunil Muthuswamy <sunilmut@...rosoft.com>
Date: Mon, 17 Jun 2019 19:26:25 +0000

> Currently, hvsock can enter into a state where epoll_wait on EPOLLOUT will
> not return even when the hvsock socket is writable, under some race
> condition. This can happen under the following sequence:
...
> Now, the EPOLLOUT will never return even if the socket write buffer is
> empty.
> 
> The fix is to set the pending size to the default size and never change it.
> This way the host will always notify the guest whenever the writable space
> is bigger than the pending size. The host is already optimized to *only*
> notify the guest when the pending size threshold boundary is crossed and
> not everytime.
> 
> This change also reduces the cpu usage somewhat since hv_stream_has_space()
> is in the hotpath of send:
> vsock_stream_sendmsg()->hv_stream_has_space()
> Earlier hv_stream_has_space was setting/clearing the pending size on every
> call.
> 
> Signed-off-by: Sunil Muthuswamy <sunilmut@...rosoft.com>
> Reviewed-by: Dexuan Cui <decui@...rosoft.com>
> ---
> - Resubmitting the patch after taking care of some spurious warnings.

Applied and queued up for -stable.