[patch V2 7/7] futex.2: Document hash preallocation opcode

[Thomas Gleixner]

At least an attempt to document the futex attached mode extension.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 man2/futex.2 |   40 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)

--- a/man2/futex.2
+++ b/man2/futex.2
@@ -767,6 +767,40 @@ operations correspond to
 and
 .BR FUTEX_WAKE_BITSET
 operations where the bit masks are all ones.
+
+.\"
+.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
+.\"
+.TP
+.BR FUTEX_PREALLOC_HASH " (since Linux ?.?.?)"
+This operation preallocates the per process private hash with a hash size
+provided by the
+.IR val
+argument.
+
+This operation tells the kernel to preallocate the per process private hash
+with a given hash size. The size is rounded up to the next power of two and
+limited internally in the kernel according to the sys control setting.
+
+The per process private hash is allocated once and cannot be resized. The
+allocation either happens by this op code or automatically when the first real
+futex operation takes place.
+
+If the hash is allocated then subsequent calls with this opcode return the
+currently active hash size.
+
+If the returned size value is 0 then the global hash is used instead of the
+process private hash. This can happen on UP machines where the private hashing
+is disabled and in case of memory allocation failure.
+
+The
+.IR uaddr ,
+.IR timeout ,
+.IR uaddr2
+and
+.I val3
+arguments are ignored.
+
 .\"
 .\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
 .\"
@@ -1334,6 +1368,12 @@ the futex word at
 Returns 0 if the caller was successfully requeued to the futex for
 the futex word at
 .IR uaddr2 .
+.TP
+.B FUTEX_PREALLOC_HASH
+Returns the size of the allocated hash, i.e. the number of hash slots. If the
+size value is 0 then the global hash is used instead of the process private
+hash. This can happen on UP machines where the private hashing is disabled and
+in case of memory allocation failure.
 .\"
 .\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
 .\"