Bug 996692

Summary: virtio-serial: write() call does not always block when host is not connected
Product: [Fedora] Fedora Reporter: Jonathan Lebon <jlebon>
Component: qemuAssignee: Fedora Virtualization Maintainers <virt-maint>
Status: CLOSED EOL QA Contact: Fedora Extras Quality Assurance <extras-qa>
Severity: unspecified Docs Contact:
Priority: unspecified    
Version: 19CC: amit.shah, berrange, cfergeau, clalancette, crobinso, dwmw2, ehabkost, extras-orphan, itamar, markmc, pbonzini, rjones, scottt.tw, virt-maint
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Last Closed: 2015-02-17 16:45:05 UTC Type: Bug
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
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Description Flags
host.c and guest.c none

Description Jonathan Lebon 2013-08-13 17:54:32 UTC
Created attachment 786228 [details]
host.c and guest.c

Description of problem:

The Guest API for virtio-serial (http://www.linux-kvm.org/page/Virtio-serial_API) indicates that a write(2) call will block if no host app is connected to the socket. However, under certain conditions, this does not happen. If a host app was INTerrupted and subsequently terminated during a read() call, the following write() call in the guest app will succeed.

Version-Release number of selected component (if applicable):

Fedora 19 x86_64 with qemu-kvm-1.4.2-4

How reproducible:

Every time

Steps to Reproduce:

The steps assume your VM is configured with a virtio-serial port named foobar.

1. Compile host.c and run on host machine, it will block on read()
2. Interrupt (^C) the application to terminate it
3. Compile guest.c and run on guest machine

Actual results:

The write() call in the guest app succeeds.

Expected results:

The write() call in the guest app blocks because no host app is connected to the socket.

Comment 1 Amit Shah 2013-08-16 08:29:40 UTC
qemu doesn't get the notification that a host-side process is terminated (and hence the socket is disconnected).  This is a limitation of the qemu's chardev layer, and a difficult one to fix in general.  This is a known problem, but we can't say when it'll be resolved.

BTW subsequent guest writes will block.

Comment 2 Fedora End Of Life 2015-01-09 19:26:27 UTC
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Comment 3 Fedora End Of Life 2015-02-17 16:45:05 UTC
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