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Co-authored-by: Jonathan Bauer <jonathan.bauer@rz.uni-freiburg.de>
Co-authored-by: Manuel Messner <mm@skelett.io>
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Using workqueues frees us from having to manage the lifecycle
of three dedicated threads. Discovery (alt server checks) and
sending keepalive packets is now done using work on the
power efficient system queue. Sending and receiving happens
via dedicated work queues with higher priority.
blk-mq has also been around for quite a while in the kernel,
so switching to it doesn't hurt backwards compatibility.
As the code is now refactored to work more as blk-mq is designed,
backwards compatibility even improved while at the same time
freeing us from an arsenal of macros that were required to make
the blk-mq port look and feel like the old implementation.
For example, the code now compiles on CentOS 7 with kernel 3.10
without requiring special macros to detect the heavily modified
RedHat kernel with all its backported features.
A few other design limitations have been rectified along the way,
e.g. switching to another server now doesn't internally disconnect
from the current one first, which theoretically could lead to a
non-working setup, if the new server isn't reachable and then -
because of some transient network error - switching back also
fails. As the discover-thread was torn down from the disconnect
call, the connection would also not repair itself eventually.
we now establish the new connection in parallel to the old one,
and only if that succeeds do we replace the old one with it,
similar to how the automatic alt-server switch already does it.
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Similar logic already exists in the fuse client:
Count how many times in a row a server was fastest when
measuring RTTs, and lower the switching threshold
more the higher the count gets.
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Convert dnbd3_host_t to struct sockaddr immediately when
adding alt servers, so we don't have to convert it every time
we establish a connection. Additionally we can now use %pISpc
in printf-like functions instead of having if/else constructs
whenever we want to print an address.
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Previously disconnect was protected against concurrent calls, but connect
wasn't. It was easy to crash the kernel when calling connect and
disconnect IOCTLs in a tight loop concurrently.
A global lock was introduced to make sure only one caller can change the
connection state at a time. dev->connection_lock needs to be aquired
when calling dnbd3_net_connect or _disconnect.
This atomic_t based locking mechanism should be turned into a mutex in a
next step, relying on mutex_trylock for cases where we don't have the
cmpxchg-schedule() loop.
Along the way it was noticed that the send/receive timeouts don't apply
to kernel_connect, which might have been the case in older 3.x kernel
versions. A crude workaround using nonblocking connect has been
introduced to emulate this, but a clean solution for this is welcomed.
Also, devices are now properly closed on module unload.
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This patch adds the feature to manually switch the dnbd3-server to a
specified server. The switching is implemented by the use of the ioctl
call SWITCH.
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This patch changes the probing logic to select the first available
dnbd3-server submitted by the ioctl OPEN call. All other specified
dnbd3-servers are added to the alternative server list and do not have
to be available.
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The ioctl OPEN call for DNBD3 devices exposed by the dnbd3 Linux kernel
module, is extended with a fixed array of dnbd3 hosts. The fixed array
allows the dnbd3-client to submit host information (IP address and port)
of multiple dnbd3-servers. This information is used to probe all
submitted dnbd3-servers and add them to the alternative dnbd3-server
list. If at least one dnbd3-server is not reachable, the OPEN ioctl call
will abort with an error code.
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This change restructures the source code directories, separates shared
form non-shared application code and adds CMake dependencies. These
dependencies allow the tracking of changes and trigger a rebuild of
those build targets where changed files are involved.
WARNING: Note that the support of the DNBD3_SERVER_AFL build option is
not supported yet. Thus, the option should be never turned on.
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