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Upgrading a lock does not play well together with acquiring an exclusive lock

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and can lead to two threads being granted exclusive access. Check that no one
has the same lock in exclusive  mode before proceeding to acquire it.

The LK_WANT_EXCL and LK_WANT_UPGRADE bits act as mini-locks and can block
other threads.  Normally this is not a problem since the mini locks are
upgraded to full locks and the release of the locks will unblock the other
threads.  However if a thread reset the bits without obtaining a full lock
other threads are not awoken. Add missing wakeups for these cases.

PR:		kern/69964
Submitted by:	Stephan Uphoff <ups at tree dot com>
Very good catch by: Stephan Uphoff <ups at tree dot com>
This commit is contained in:
Alexander Kabaev 2004-08-16 15:01:22 +00:00
parent f03b27517b
commit c8b876219f
1 changed files with 12 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

17
sys/kern/kern_lock.c
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@ -335,8 +335,12 @@ debuglockmgr(lkp, flags, interlkp, td, name, file, line)
error = acquire(&lkp, extflags, LK_SHARE_NONZERO);
lkp->lk_flags &= ~LK_WANT_UPGRADE;
if (error)
break;
if (error) {
if ((lkp->lk_flags & ( LK_WANT_EXCL | LK_WAIT_NONZERO)) == (LK_WANT_EXCL | LK_WAIT_NONZERO))
wakeup((void *)lkp);
break;
}
lkp->lk_flags |= LK_HAVE_EXCL;
lkp->lk_lockholder = thr;
if (lkp->lk_exclusivecount != 0)
@ -382,17 +386,20 @@ debuglockmgr(lkp, flags, interlkp, td, name, file, line)
/*
* Try to acquire the want_exclusive flag.
*/
error = acquire(&lkp, extflags, (LK_HAVE_EXCL | LK_WANT_EXCL));
error = acquire(&lkp, extflags, LK_WANT_EXCL);
if (error)
break;
lkp->lk_flags |= LK_WANT_EXCL;
/*
* Wait for shared locks and upgrades to finish.
*/
error = acquire(&lkp, extflags, LK_WANT_UPGRADE | LK_SHARE_NONZERO);
error = acquire(&lkp, extflags, LK_HAVE_EXCL | LK_WANT_UPGRADE | LK_SHARE_NONZERO);
lkp->lk_flags &= ~LK_WANT_EXCL;
if (error)
if (error) {
if (lkp->lk_flags & LK_WAIT_NONZERO)
wakeup((void *)lkp);
break;
}
lkp->lk_flags |= LK_HAVE_EXCL;
lkp->lk_lockholder = thr;
if (lkp->lk_exclusivecount != 0)