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SubjectLocking scheme to block less
Currently, the kernel uses only spin_locks, which are similar to mutex 
locks; locks constrict operations to being mutually exclusive. There
are many cases where a set of three or more operations has two or more
which can occur concurrently, and one or more which cannot occur
concurrently with other operations. These are normally handled with
read-write locks.

If the kernel provided a read-write locking semaphore, the time spent
blocking could be greatly reduced in a safe and effective manner. To
maximize efficiency, the following functions would be needed:

spin_read_lock(spin_rwlock_t *lock);
Read lock a spin_rwlock_t. Multiple concurrent read locks can be
held, and will block write locks. A write lock blocks read locks.

spin_write_lock(spin_rwlock_t *lock);
Write lock a spin_rwlock_t. Only one write lock can be held at any one
time, and no read lock may be held during write locks. Held read locks
block write locks; held write locks block read locks.

spin_read_to_write_lock(spin_rwlock_t *lock);
This is a read lock that will become a write lock. It allows other read
locks; but blocks write locks and other read_to_write locks. It is
blocked by write locks.

In the course of kernel execution, some functions will wish to read
through a linked list, find a point to insert or remove data, and then
perform the operation. Two such operations will race for a write lock
when they find the point in the list they wish to alter. If the points
are significantly close-- for example, insertion after an index to be
deleted, or both inserting after a given index-- then a race condition
occurs, creating an inconsistent internal state and a possible reference
to freed memory.

A read_to_write lock will block two such operations from occuring
concurrently, while still allowing read only operations AND still being
blocked when switched to write mode by both read and write operations.

spin_r2w_write(spin_rwlock_t *lock);
Exchange currently held read_to_write lock for a write lock. Same rules
as a write lock apply; this operation is blocked by read locks and
blocks all locks.

spin_rw_unlock(spin_rwlock_t *lock);
Unlocks a held spin_rwlockt_lock.

I believe that these locking semaphores would be generically useful in
reducing the time spent blocking in the kernel.

Notice that these are similar to pthreads rwlocks; except that there is
a read_to_write lock specified. This is not userspace, this is a
kernel; nobody cares about standards internally, but everyone hates races.

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