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author | Stefan Hajnoczi <stefanha@redhat.com> | 2014-03-03 11:30:03 +0100 |
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committer | Stefan Hajnoczi <stefanha@redhat.com> | 2014-03-13 14:42:21 +0100 |
commit | 2da61b671eb89fcaa306738f44eed472977d6587 (patch) | |
tree | 61673e48ab889a6ea113ff4d2be2d48d7108d1a1 /include/qemu | |
parent | 11f590b1a242492a0108da42f40f0e2b20f0a778 (diff) | |
download | hqemu-2da61b671eb89fcaa306738f44eed472977d6587.zip hqemu-2da61b671eb89fcaa306738f44eed472977d6587.tar.gz |
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>
Diffstat (limited to 'include/qemu')
-rw-r--r-- | include/qemu/rfifolock.h | 54 |
1 files changed, 54 insertions, 0 deletions
diff --git a/include/qemu/rfifolock.h b/include/qemu/rfifolock.h new file mode 100644 index 0000000..b23ab53 --- /dev/null +++ b/include/qemu/rfifolock.h @@ -0,0 +1,54 @@ +/* + * Recursive FIFO lock + * + * Copyright Red Hat, Inc. 2013 + * + * Authors: + * Stefan Hajnoczi <stefanha@redhat.com> + * + * This work is licensed under the terms of the GNU GPL, version 2 or later. + * See the COPYING file in the top-level directory. + * + */ + +#ifndef QEMU_RFIFOLOCK_H +#define QEMU_RFIFOLOCK_H + +#include "qemu/thread.h" + +/* Recursive FIFO lock + * + * This lock provides more features than a plain mutex: + * + * 1. Fairness - enforces FIFO order. + * 2. Nesting - can be taken recursively. + * 3. Contention callback - optional, called when thread must wait. + * + * The recursive FIFO lock is heavyweight so prefer other synchronization + * primitives if you do not need its features. + */ +typedef struct { + QemuMutex lock; /* protects all fields */ + + /* FIFO order */ + unsigned int head; /* active ticket number */ + unsigned int tail; /* waiting ticket number */ + QemuCond cond; /* used to wait for our ticket number */ + + /* Nesting */ + QemuThread owner_thread; /* thread that currently has ownership */ + unsigned int nesting; /* amount of nesting levels */ + + /* Contention callback */ + void (*cb)(void *); /* called when thread must wait, with ->lock + * held so it may not recursively lock/unlock + */ + void *cb_opaque; +} RFifoLock; + +void rfifolock_init(RFifoLock *r, void (*cb)(void *), void *opaque); +void rfifolock_destroy(RFifoLock *r); +void rfifolock_lock(RFifoLock *r); +void rfifolock_unlock(RFifoLock *r); + +#endif /* QEMU_RFIFOLOCK_H */ |