diff options
| author |   art <art@openbsd.org> | 2007-04-04 18:01:57 +0000 |
|---|---|---|
| committer | art <art@openbsd.org> | 2007-04-04 18:01:57 +0000 |
| commit | 4e4e01ffb7937d2d5d0a04824c5a94363a89ab63 (patch) | |
| tree | 100739617f5c2826d7301ca9d7f6a6efb8bae83e | |
| parent | Mechanically rename the "flags" and "version" fields in struct vm_page (diff) | |
| download | wireguard-openbsd-4e4e01ffb7937d2d5d0a04824c5a94363a89ab63.tar.xz wireguard-openbsd-4e4e01ffb7937d2d5d0a04824c5a94363a89ab63.zip | |
Implement RW_DOWNGRADE that downgrades an exclusive lock to a shared lock
without letting any other exclusive locks in between. As opposed to upgrading
locks, this is easy and solves real problems.
deraadt@ ok
| -rw-r--r-- | sys/kern/kern_rwlock.c | 38 | ||||
| -rw-r--r-- | sys/sys/rwlock.h | 6 |
2 files changed, 35 insertions, 9 deletions
diff --git a/sys/kern/kern_rwlock.c b/sys/kern/kern_rwlock.c index 1d5bb954787..85f1e6ace92 100644 --- a/sys/kern/kern_rwlock.c +++ b/sys/kern/kern_rwlock.c @@ -1,4 +1,4 @@ -/* $OpenBSD: kern_rwlock.c,v 1.9 2006/11/30 20:08:22 mk Exp $ */ +/* $OpenBSD: kern_rwlock.c,v 1.10 2007/04/04 18:01:57 art Exp $ */ /* * Copyright (c) 2002, 2003 Artur Grabowski <art@openbsd.org> @@ -67,6 +67,13 @@ static const struct rwlock_op { 0, PLOCK }, + { /* RW_DOWNGRADE */ + RWLOCK_READ_INCR - RWLOCK_WRLOCK, + 0, + 0, + -1, + PLOCK + }, }; #ifndef __HAVE_MD_RWLOCK @@ -138,6 +145,16 @@ rw_enter_diag(struct rwlock *rwl, int flags) if (RW_PROC(curproc) == RW_PROC(rwl->rwl_owner)) panic("rw_enter: locking against myself"); break; + case RW_DOWNGRADE: + /* + * If we're downgrading, we must hold the write lock. + */ + if ((rwl->rwl_owner & RWLOCK_WRLOCK) == 0) + panic("rw_enter: downgrade of non-write lock"); + if (RW_PROC(curproc) != RW_PROC(rwl->rwl_owner)) + panic("rw_enter: downgrade, not holder"); + break; + default: panic("rw_enter: unknown op 0x%x", flags); } @@ -154,9 +171,6 @@ rw_init(struct rwlock *rwl, const char *name) rwl->rwl_name = name; } -/* - * You are supposed to understand this. - */ int rw_enter(struct rwlock *rwl, int flags) { @@ -167,14 +181,15 @@ rw_enter(struct rwlock *rwl, int flags) op = &rw_ops[flags & RW_OPMASK]; inc = op->inc + RW_PROC(curproc) * op->proc_mult; - prio = op->wait_prio; - if (flags & RW_INTR) - prio |= PCATCH; retry: while (__predict_false(((o = rwl->rwl_owner) & op->check) != 0)) { if (rw_test_and_set(&rwl->rwl_owner, o, o | op->wait_set)) continue; + prio = op->wait_prio; + if (flags & RW_INTR) + prio |= PCATCH; + rw_enter_diag(rwl, flags); if (flags & RW_NOSLEEP) @@ -188,6 +203,15 @@ retry: if (__predict_false(rw_test_and_set(&rwl->rwl_owner, o, o + inc))) goto retry; + /* + * If old lock had RWLOCK_WAIT and RWLOCK_WRLOCK set, it means we + * downgraded a write lock and had possible read waiter, wake them + * to let them retry the lock. + */ + if (__predict_false((o & (RWLOCK_WRLOCK|RWLOCK_WAIT)) == + (RWLOCK_WRLOCK|RWLOCK_WAIT))) + wakeup(rwl); + return (0); } diff --git a/sys/sys/rwlock.h b/sys/sys/rwlock.h index cb89d50038a..405b9d9351d 100644 --- a/sys/sys/rwlock.h +++ b/sys/sys/rwlock.h @@ -1,4 +1,4 @@ -/* $OpenBSD: rwlock.h,v 1.8 2006/06/05 05:15:22 tedu Exp $ */ +/* $OpenBSD: rwlock.h,v 1.9 2007/04/04 18:01:57 art Exp $ */ /* * Copyright (c) 2002 Artur Grabowski <art@openbsd.org> * All rights reserved. @@ -99,7 +99,9 @@ int rw_enter(struct rwlock *, int); void rw_exit(struct rwlock *); #define RW_WRITE 0x00UL /* exclusive lock */ #define RW_READ 0x01UL /* shared lock */ -#define RW_OPMASK 0x01UL +#define RW_DOWNGRADE 0x02UL /* downgrade exclusive to shared */ +#define RW_OPMASK 0x03UL + #define RW_INTR 0x10UL /* interruptible sleep */ #define RW_SLEEPFAIL 0x20UL /* fail if we slept for the lock */ #define RW_NOSLEEP 0x40UL /* don't wait for the lock */ |