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H A D | splice.c | ee5e0011 Thu Feb 07 09:45:19 CST 2019 Slavomir Kaslev <kaslevs@vmware.com> fs: Make splice() and tee() take into account O_NONBLOCK flag on pipes
The current implementation of splice() and tee() ignores O_NONBLOCK set on pipe file descriptors and checks only the SPLICE_F_NONBLOCK flag for blocking on pipe arguments. This is inconsistent since splice()-ing from/to non-pipe file descriptors does take O_NONBLOCK into consideration.
Fix this by promoting O_NONBLOCK, when set on a pipe, to SPLICE_F_NONBLOCK.
Some context for how the current implementation of splice() leads to inconsistent behavior. In the ongoing work[1] to add VM tracing capability to trace-cmd we stream tracing data over named FIFOs or vsockets from guests back to the host.
When we receive SIGINT from user to stop tracing, we set O_NONBLOCK on the input file descriptor and set SPLICE_F_NONBLOCK for the next call to splice(). If splice() was blocked waiting on data from the input FIFO, after SIGINT splice() restarts with the same arguments (no SPLICE_F_NONBLOCK) and blocks again instead of returning -EAGAIN when no data is available.
This differs from the splice() behavior when reading from a vsocket or when we're doing a traditional read()/write() loop (trace-cmd's --nosplice argument).
With this patch applied we get the same behavior in all situations after setting O_NONBLOCK which also matches the behavior of doing a read()/write() loop instead of splice().
This change does have potential of breaking users who don't expect EAGAIN from splice() when SPLICE_F_NONBLOCK is not set. OTOH programs that set O_NONBLOCK and don't anticipate EAGAIN are arguably buggy[2].
[1] https://github.com/skaslev/trace-cmd/tree/vsock [2] https://github.com/torvalds/linux/blob/d47e3da1759230e394096fd742aad423c291ba48/fs/read_write.c#L1425
Signed-off-by: Slavomir Kaslev <kaslevs@vmware.com> Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> ee5e0011 Thu Feb 07 09:45:19 CST 2019 Slavomir Kaslev <kaslevs@vmware.com> fs: Make splice() and tee() take into account O_NONBLOCK flag on pipes The current implementation of splice() and tee() ignores O_NONBLOCK set on pipe file descriptors and checks only the SPLICE_F_NONBLOCK flag for blocking on pipe arguments. This is inconsistent since splice()-ing from/to non-pipe file descriptors does take O_NONBLOCK into consideration. Fix this by promoting O_NONBLOCK, when set on a pipe, to SPLICE_F_NONBLOCK. Some context for how the current implementation of splice() leads to inconsistent behavior. In the ongoing work[1] to add VM tracing capability to trace-cmd we stream tracing data over named FIFOs or vsockets from guests back to the host. When we receive SIGINT from user to stop tracing, we set O_NONBLOCK on the input file descriptor and set SPLICE_F_NONBLOCK for the next call to splice(). If splice() was blocked waiting on data from the input FIFO, after SIGINT splice() restarts with the same arguments (no SPLICE_F_NONBLOCK) and blocks again instead of returning -EAGAIN when no data is available. This differs from the splice() behavior when reading from a vsocket or when we're doing a traditional read()/write() loop (trace-cmd's --nosplice argument). With this patch applied we get the same behavior in all situations after setting O_NONBLOCK which also matches the behavior of doing a read()/write() loop instead of splice(). This change does have potential of breaking users who don't expect EAGAIN from splice() when SPLICE_F_NONBLOCK is not set. OTOH programs that set O_NONBLOCK and don't anticipate EAGAIN are arguably buggy[2]. [1] https://github.com/skaslev/trace-cmd/tree/vsock [2] https://github.com/torvalds/linux/blob/d47e3da1759230e394096fd742aad423c291ba48/fs/read_write.c#L1425 Signed-off-by: Slavomir Kaslev <kaslevs@vmware.com> Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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