Couple minor notes after upgrading to 23.02

[Norman Feske]

Hi Cedrik,

thanks for sharing your findings.

On 2023-06-29 16:44, ttcoder at netcourrier.com wrote:
> * vfs_pipe: components that use the "pipe" plug-in should make sure to enable RTC (<libc rtc="/dev/rtc"...>)
> as of 23.02 -- previously it would work without specifying an RTC path. This probably has zero impact
> since everyone properly configures their libc config (unlike me in the one scenario that brought this up..!)

This made me curios because the nightly tests [1,2] for the vfs_pipe 
plugin do in fact not configure the rtc for the libc, yet are known to work.

[1] 
https://github.com/genodelabs/genode/blob/master/repos/libports/recipes/pkg/test-libc_fifo_pipe/runtime
[2] 
https://github.com/genodelabs/genode/blob/master/repos/libports/recipes/pkg/test-libc_pipe/runtime

I guess that the use of the rtc has a side effect that covers up the 
symptom of a problem that might still lurk there. To get to the bottom 
of it, I'd need more information or - ideally - a scenario to reproduce 
the non-working state.

There is one suspicion: The VFS changes in 23.02 foster the batching of 
I/O in the file-system session. When operating both sides of a pipe 
through the same file-system session, there is the risk that a write 
operations fits into the file-system session's packet-stream buffer but 
not in the pipe buffer in the remote pipe. Once the pipe is saturated, a 
reader must consume content before any new progress can be made. But 
when the reader issues its read request through the same file-system 
session as used by the writer - which is still clogged by the not yet 
completed write operation - we end up in a deadlock situation. May this 
be the case in your scenario?

In practice, such situations don't occur whenever both ends of one pipe 
are operated by different components - each using a distinct file-system 
session. But when interacting with a pipe behind a chain of VFS servers, 
stalling effects are plausible. But as I said, I'm just speculating.

Should you by any chance find a way to come up with a run script that 
shows the non-working behavior, I'd love to investigate it.

Cheers
Norman

-- 
Dr.-Ing. Norman Feske
Genode Labs

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