From: Linus Torvalds <firstname.lastname@example.org> To: "Alex Xu (Hello71)" <email@example.com> Cc: Linux Kernel Mailing List <firstname.lastname@example.org>, David Howells <email@example.com>, firstname.lastname@example.org, Rasmus Villemoes <email@example.com>, Greg Kroah-Hartman <firstname.lastname@example.org>, Peter Zijlstra <email@example.com>, Nicolas Dichtel <firstname.lastname@example.org>, Ian Kent <email@example.com>, Christian Brauner <firstname.lastname@example.org>, email@example.com, firstname.lastname@example.org, linux-block <email@example.com>, LSM List <firstname.lastname@example.org>, linux-fsdevel <email@example.com>, Linux API <firstname.lastname@example.org> Subject: Re: [REGRESSION?] Simultaneous writes to a reader-less, non-full pipe can hang Date: Wed, 4 Aug 2021 09:31:01 -0700 [thread overview] Message-ID: <CAHk-=wiLr55zHUWNzmp3DeoO0DUaYp7vAzQB5KUCni5FpwC7Uw@mail.gmail.com> (raw) In-Reply-To: <1628086770.5rn8p04n6j.none@localhost> Your program is buggy. On Wed, Aug 4, 2021 at 8:37 AM Alex Xu (Hello71) <email@example.com> wrote: > > pipe(pipefd); > printf("init buffer: %d\n", fcntl(pipefd, F_GETPIPE_SZ)); > printf("new buffer: %d\n", fcntl(pipefd, F_SETPIPE_SZ, 0)); Yeah, what did you expect this to do? You said you want a minimal buffer, you get a really small buffer. Then you try to write multiple messages to the pipe that you just said should have a minimum size. Don't do that then. > /proc/x/stack shows that the remaining thread is hanging at pipe.c:560. > It looks like not only there needs to be space in the pipe, but also > slots. Correct. The fullness of a pipe is not about whether it has the possibility of merging more bytes into an existing not-full slot, but about whether it has empty slots left. Part of that is simply the POSIX pipe guarantees - a write of size PIPE_BUF or less is guaranteed to be atomic, so it mustn't be split among buffers. So a pipe must not be "writable" unless it has space for at least that much (think select/poll, which don't know the size of the write). The fact that we might be able to reuse a partially filled buffer for smaller writes is simply not relevant to that issue. And yes, we could have different measures of "could write" for different writes, but we just don't have or want that complexity. Please don't mess with F_SETPIPE_SZ unless you have a really good reason to do so, and actually understand what you are doing. Doing a F_SETPIPE_SZ, 0 basically means "I want the mimimum pipe size possible". And that one accepts exactly one write at a time. Of course, the exact semantics are much more complicated than that "exactly one write". The pipe write code will optimistically merge writes into a previous buffer, which means that depending on the pattern of your writes, the exact number of bytes you can write will be very different. But that "merge writes into a previous buffer" only appends to the buffer - not _reuse_ it - so when each buffer is one page in size, what happens is that you can merge up to 4096 bytes worth of writes, but then after that the pipe write will want a new buffer - even if the old buffer is now empty because of old reads. That's why your test program won't block immediately: both writers will actually start out happily doing writes into that one buffer that is allocated, but at some point that buffer ends, and it wants to allocate a new buffer. But you told it not to allocate more buffers, and the old buffer is never completely empty because your readers never read _everythign_, so it will hang, waiting for you to empty the one minimal buffer it allocated. And that will never happen. There's a very real reason why we do *not* by default say "pipes can only ever use only one buffer". I don't think this is a regression, but if you have an actual application - not a test program - that does crazy things like this and used to work (I'm not sure it has ever worked, though), we can look into making it work again. That said, I suspect the way to make it work is to just say "the minimum pipe size is two slots" rather than change the "we want at least one empty slot". Exactly because of that whole "look, we must not consider a pipe that doesn't have a slot writable". Because clearly people don't understand how subtle F_SETPIPE_SZ is. It's not really a "byte count", even though that is how it's expressed. Linus
next prev parent reply other threads:[~2021-08-04 16:31 UTC|newest] Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top [not found] <1628086770.5rn8p04n6j.none.ref@localhost> 2021-08-04 15:37 ` Alex Xu (Hello71) 2021-08-04 16:31 ` Linus Torvalds [this message] 2021-08-04 19:48 ` Alex Xu (Hello71) 2021-08-04 20:04 ` Linus Torvalds
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