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B.6. Glock Tracepoints
The tracepoints are also designed to be able to confirm the correctness of the cache control by combining them with the
blktraceoutput and with knowledge of the on-disk layout. It is then possible to check that any given I/O has been issued and completed under the correct lock, and that no races are present.
gfs2_glock_state_changetracepoint is the most important one to understand. It tracks every state change of the glock from initial creation right through to the final demotion which ends with
gfs2_glock_putand the final NL to unlocked transition. The l (locked) glock flag is always set before a state change occurs and will not be cleared until after it has finished. There are never any granted holders (the H glock holder flag) during a state change. If there are any queued holders, they will always be in the W (waiting) state. When the state change is complete then the holders may be granted which is the final operation before the l glock flag is cleared.
gfs2_demote_rqtracepoint keeps track of demote requests, both local and remote. Assuming that there is enough memory on the node, the local demote requests will rarely be seen, and most often they will be created by
umountor by occasional memory reclaim. The number of remote demote requests is a measure of the contention between nodes for a particular inode or resource group.
gfs2_glock_lock_timetracepoint provides information on the time taken by requests to the DLM. The blocking (
b) flag was introduced into the glock specifically to be used in combination with this tracepoint.
When a holder is granted a lock,
gfs2_promoteis called, this occurs as the final stages of a state change or when a lock is requested which can be granted immediately due to the glock state already caching a lock of a suitable mode. If the holder is the first one to be granted for this glock, then the f (first) flag is set on that holder. This is currently used only by resource groups.