Date: Tue, 29 Oct 2002 01:54:26 -0800 (PST) From: Neal Fachan <neal@isilon.com> To: freebsd-gnats-submit@FreeBSD.org Subject: kern/44585: bug in lockmgr Message-ID: <200210290954.g9T9sQ9P050011@www.freebsd.org>
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>Number: 44585
>Category: kern
>Synopsis: bug in lockmgr
>Confidential: no
>Severity: serious
>Priority: low
>Responsible: freebsd-bugs
>State: open
>Quarter:
>Keywords:
>Date-Required:
>Class: sw-bug
>Submitter-Id: current-users
>Arrival-Date: Tue Oct 29 02:00:03 PST 2002
>Closed-Date:
>Last-Modified:
>Originator: Neal Fachan
>Release: 5.0-CURRENT from a few months back
>Organization:
Isilon Systems, Inc.
>Environment:
>Description:
Imagine three threads.
Threads 1 and 2 get shared lock:
lk_sharecount = 2;
lk_flags = LK_SHARE_NONZERO;
Thread 3 goes to get exclusive. It gets through the first acquire
without sleeping (LK_HAVE_EXCL | LK_WANT_EXCL). It then sets
LK_WANT_EXCL and then sleeps on the second acquire:
lk_sharecount = 2;
lk_flags = LK_WANT_EXCL | LK_SHARE_NONZERO;
thread 3 sleeping on: !(LK_WANT_UPGRADE | LK_SHARE_NONZERO);
Thread 2 goes to sleep waiting on an upgrade:
lk_sharecount = 1;
lk_flags = LK_WANT_EXCL | LK_WANT_UPGRADE | LK_SHARE_NONZERO;
thread 2 sleeping on: !(LK_SHARE_NONZERO);
thread 3 sleeping on: !(LK_SHARE_NONZERO | LK_WANT_UPGRADE);
Thread 1 unlocks shared lock:
lk_sharecount = 0;
lk_flags = LK_WANT_EXCL | LK_WANT_UPGRADE;
thread 2 sleeping on: !(LK_SHARE_NONZERO);
thread 3 sleeping on: !(LK_SHARE_NONZERO | LK_WANT_UPGRADE);
Thread 2 wakes up first, and takes exclusive lock:
lk_exclusivecount = 1;
lk_flags = LK_WANT_EXCL | LK_HAVE_EXCL;
thread 3 sleeping on: !(LK_SHARE_NONZERO | LK_WANT_UPGRADE);
Thread 3 wakes up second, and also tries to take exclusive lock:
lk_exclusivecount = 2; /* !!! big trouble for moose and squirrel */
lk_flags = LK_HAVE_EXCL;
>How-To-Repeat:
>Fix:
The fix is to put LK_HAVE_EXCL in the second acquire of the LK_EXCLUSIVE
path: change acquire(LK_WANT_UPGRADE | LK_SHARE_NONZERO) to
acquire(LK_HAS_EXCL | LK_WANT_UPGRADE | LK_SHARE_NONZERO).
>Release-Note:
>Audit-Trail:
>Unformatted:
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