Date: Wed, 04 Mar 2026 19:15:53 +0100 From: "Christos Margiolis" <christos@freebsd.org> To: "Unitrunker" <unitrunker@unitrunker.net> Cc: <freebsd-multimedia@freebsd.org> Subject: Re: cuse(3) cuse_wait_and_process thread safety Message-ID: <DGU7BRMO0NSM.1R59RB18HVL88@freebsd.org> In-Reply-To: <z4gb0FHta2RgVVOgb9R1aetw5B-mIlUajdokq7xGi4thxQVtsIcZkCTsL4o6WzZbgwx4qy6b4LQoc_mulgDaiPV2BvRL08z-tW-eZLipmdc=@unitrunker.net>
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On Wed Feb 18, 2026 at 2:14 PM CET, Unitrunker wrote: > I'm posting this to the multimedia list because virtual_oss is the > main consumer of cuse(3). > > Looking over the documentation for cuse_wait_and_process, cuse.3 > reads: > > int cuse_wait_and_process(void) This function will block and do event > processing. If parallel I/O is required multiple threads must be created > looping on this function. This function returns 0 on success or a > negative value on failure. See CUSE_ERR_XXX for known error codes. > > My concern lies in the suggestion to provide parallel I/O by having > multiple threads call this function. > > I'm writing this message to get some extra eyes on this and maybe some > feedback. > > cuse_wait_and_process processes exactly one I/O operation and returns. > No more and no less. > > If the cuse driver is holding two pending IO operations for two > different cuse devices, it isn't so important as to which goes first. > They can race and not interfere with each other. > > However, if there are two or more pending IO operations - such as two > reads or two writes to the same device and each operation is picked up > be a different thread at more or less the same time, the second IO > operation might finish before the first. In the case of virtual_oss, > this might cause out-of-order buffering of audio data. > > Today, virtual_oss creates four threads per virtual device (see > "create_threads" in usr.sbin/virtual_oss/virtual_oss/main.c). I see a > potential problem. > > I'll be very happy if this turns out to be a non-issue. > > Regards, > Rick I'm the maintainer of virtual_oss. To be fair, I don't know what was HPS's (RIP) rationale for giving each device 4 threads, so I'm not sure how to answer this question. Have you run into the problem you described since you sent the email? Christoshome | help
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