tree 61673e48ab889a6ea113ff4d2be2d48d7108d1a1
parent 11f590b1a242492a0108da42f40f0e2b20f0a778
author Stefan Hajnoczi <stefanha@redhat.com> 1393842603 +0100
committer Stefan Hajnoczi <stefanha@redhat.com> 1394718141 +0100

rfifolock: add recursive FIFO lock

QemuMutex does not guarantee fairness and cannot be acquired
recursively:

Fairness means each locker gets a turn and the scheduler cannot cause
starvation.

Recursive locking is useful for composition, it allows a sequence of
locking operations to be invoked atomically by acquiring the lock around
them.

This patch adds RFifoLock, a recursive lock that guarantees FIFO order.
Its first user is added in the next patch.

RFifoLock has one additional feature: it can be initialized with an
optional contention callback.  The callback is invoked whenever a thread
must wait for the lock.  For example, it can be used to poke the current
owner so that they release the lock soon.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>