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[ Upstream commit cb359b60416701c8bed82fec79de25a144beb893 ] 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: - fd = socket(hvsocket) - fd_out = dup(fd) - fd_in = dup(fd) - start a writer thread that writes data to fd_out with a combination of epoll_wait(fd_out, EPOLLOUT) and - start a reader thread that reads data from fd_in with a combination of epoll_wait(fd_in, EPOLLIN) - On the host, there are two threads that are reading/writing data to the hvsocket stack: hvs_stream_has_space hvs_notify_poll_out vsock_poll sock_poll ep_poll Race condition: check for epollout from ep_poll(): assume no writable space in the socket hvs_stream_has_space() returns 0 check for epollin from ep_poll(): assume socket has some free space < HVS_PKT_LEN(HVS_SEND_BUF_SIZE) hvs_stream_has_space() will clear the channel pending send size host will not notify the guest because the pending send size has been cleared and so the hvsocket will never mark the socket writable 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@microsoft.com> Reviewed-by: Dexuan Cui <decui@microsoft.com> Signed-off-by: David S. Miller <davem@davemloft.net> Signed-off-by: Sasha Levin <sashal@kernel.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.