xfs: drop the obsolete comment on filestream locking

/*

 * Allocation group filestream associations are tracked with per-ag atomic

 * counters.  These counters allow xfs_filestream_pick_ag() to tell whether a

 * particular AG already has active filestreams associated with it. The mount

 * point's m_peraglock is used to protect these counters from per-ag array

 * re-allocation during a growfs operation.  When xfs_growfs_data_private() is

 * about to reallocate the array, it calls xfs_filestream_flush() with the

 * m_peraglock held in write mode.

 *

 * Since xfs_mru_cache_flush() guarantees that all the free functions for all

 * the cache elements have finished executing before it returns, it's safe for

 * the free functions to use the atomic counters without m_peraglock protection.

 * This allows the implementation of xfs_fstrm_free_func() to be agnostic about

 * whether it was called with the m_peraglock held in read mode, write mode or

 * not held at all.  The race condition this addresses is the following:

 *

 *  - The work queue scheduler fires and pulls a filestream directory cache

 *    element off the LRU end of the cache for deletion, then gets pre-empted.

 *  - A growfs operation grabs the m_peraglock in write mode, flushes all the

 *    remaining items from the cache and reallocates the mount point's per-ag

 *    array, resetting all the counters to zero.

 *  - The work queue thread resumes and calls the free function for the element

 *    it started cleaning up earlier.  In the process it decrements the

 *    filestreams counter for an AG that now has no references.

 *

 * With a shrinkfs feature, the above scenario could panic the system.

 *

 * All other uses of the following macros should be protected by either the

 * m_peraglock held in read mode, or the cache's internal locking exposed by the

 * interval between a call to xfs_mru_cache_lookup() and a call to

 * xfs_mru_cache_done().  In addition, the m_peraglock must be held in read mode

 * when new elements are added to the cache.

 *

 * Combined, these locking rules ensure that no associations will ever exist in

 * the cache that reference per-ag array elements that have since been

 * reallocated.

 * particular AG already has active filestreams associated with it.

 */

int

xfs_filestream_peek_ag(