A lock can be contended, which is when, a context wants to acquire the lock but it has already been taken, so it must wait for the unlock to occur. Heavy contention can create severe performance bottlenecks; the kernel provides lock statistics with a view to easily identifying heavily contended locks. Enable lock statistics by turning on the CONFIG_LOCK_STAT kernel configuration option (without this, the /proc/lock_stat entry will not be present, the typical case on most distribution kernels).
The lock stats code takes advantage of the fact that lockdep inserts hooks into the locking code path (the __contended, __acquired, and __released hooks) to gather statistics at these crucial points. The neatly written kernel documentation on lock statistics (https://www.kernel.org/doc/html/latest/locking/lockstat.html#lock-statistics) conveys this information (and a lot more) with a useful state diagram; do look it up.
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