2da61b671e
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>
55 lines
1.6 KiB
C
55 lines
1.6 KiB
C
/*
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* Recursive FIFO lock
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*
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* Copyright Red Hat, Inc. 2013
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*
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* Authors:
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* Stefan Hajnoczi <stefanha@redhat.com>
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*
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* This work is licensed under the terms of the GNU GPL, version 2 or later.
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* See the COPYING file in the top-level directory.
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*
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*/
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#ifndef QEMU_RFIFOLOCK_H
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#define QEMU_RFIFOLOCK_H
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#include "qemu/thread.h"
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/* Recursive FIFO lock
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*
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* This lock provides more features than a plain mutex:
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*
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* 1. Fairness - enforces FIFO order.
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* 2. Nesting - can be taken recursively.
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* 3. Contention callback - optional, called when thread must wait.
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*
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* The recursive FIFO lock is heavyweight so prefer other synchronization
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* primitives if you do not need its features.
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*/
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typedef struct {
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QemuMutex lock; /* protects all fields */
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/* FIFO order */
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unsigned int head; /* active ticket number */
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unsigned int tail; /* waiting ticket number */
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QemuCond cond; /* used to wait for our ticket number */
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/* Nesting */
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QemuThread owner_thread; /* thread that currently has ownership */
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unsigned int nesting; /* amount of nesting levels */
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/* Contention callback */
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void (*cb)(void *); /* called when thread must wait, with ->lock
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* held so it may not recursively lock/unlock
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*/
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void *cb_opaque;
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} RFifoLock;
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void rfifolock_init(RFifoLock *r, void (*cb)(void *), void *opaque);
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void rfifolock_destroy(RFifoLock *r);
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void rfifolock_lock(RFifoLock *r);
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void rfifolock_unlock(RFifoLock *r);
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#endif /* QEMU_RFIFOLOCK_H */
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