bca5b06580
md-cluster uses MD_CLUSTER_SEND_LOCK to make node can exclusively send msg.
During sending msg, node can concurrently receive msg from another node.
When node does resync job, grab token_lockres:EX may trigger a deadlock:
```
nodeA nodeB
-------------------- --------------------
a.
send METADATA_UPDATED
held token_lockres:EX
b.
md_do_sync
resync_info_update
send RESYNCING
+ set MD_CLUSTER_SEND_LOCK
+ wait for holding token_lockres:EX
c.
mdadm /dev/md0 --remove /dev/sdg
+ held reconfig_mutex
+ send REMOVE
+ wait_event(MD_CLUSTER_SEND_LOCK)
d.
recv_daemon //METADATA_UPDATED from A
process_metadata_update
+ (mddev_trylock(mddev) ||
MD_CLUSTER_HOLDING_MUTEX_FOR_RECVD)
//this time, both return false forever
```
Explaination:
a. A send METADATA_UPDATED
This will block another node to send msg
b. B does sync jobs, which will send RESYNCING at intervals.
This will be block for holding token_lockres:EX lock.
c. B do "mdadm --remove", which will send REMOVE.
This will be blocked by step <b>: MD_CLUSTER_SEND_LOCK is 1.
d. B recv METADATA_UPDATED msg, which send from A in step <a>.
This will be blocked by step <c>: holding mddev lock, it makes
wait_event can't hold mddev lock. (btw,
MD_CLUSTER_HOLDING_MUTEX_FOR_RECVD keep ZERO in this scenario.)
There is a similar deadlock in commit
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README
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.