Date: Tue, 27 Jan 2009 16:25:45 +0000 (UTC) From: John Baldwin <jhb@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-stable@freebsd.org, svn-src-stable-7@freebsd.org Subject: svn commit: r187776 - in stable/7/sys: . contrib/pf dev/ath/ath_hal dev/cxgb ufs/ufs Message-ID: <200901271625.n0RGPj6Z090896@svn.freebsd.org>
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Author: jhb Date: Tue Jan 27 16:25:45 2009 New Revision: 187776 URL: http://svn.freebsd.org/changeset/base/187776 Log: MFC: Add a comment explaining why the "bufwait" / "dirhash" LOR reported by WITNESS will not actually result in a deadlock. Modified: stable/7/sys/ (props changed) stable/7/sys/contrib/pf/ (props changed) stable/7/sys/dev/ath/ath_hal/ (props changed) stable/7/sys/dev/cxgb/ (props changed) stable/7/sys/ufs/ufs/ufs_dirhash.c Modified: stable/7/sys/ufs/ufs/ufs_dirhash.c ============================================================================== --- stable/7/sys/ufs/ufs/ufs_dirhash.c Tue Jan 27 16:12:19 2009 (r187775) +++ stable/7/sys/ufs/ufs/ufs_dirhash.c Tue Jan 27 16:25:45 2009 (r187776) @@ -126,6 +126,18 @@ static struct mtx ufsdirhash_mtx; * free a dirhash structure that was recycled by ufsdirhash_recycle(). * * The dirhash lock may be held across io operations. + * + * WITNESS reports a lock order reversal between the "bufwait" lock + * and the "dirhash" lock. However, this specific reversal will not + * cause a deadlock. To get a deadlock, one would have to lock a + * buffer followed by the dirhash while a second thread locked a + * buffer while holding the dirhash lock. The second order can happen + * under a shared or exclusive vnode lock for the associated directory + * in lookup(). The first order, however, can only happen under an + * exclusive vnode lock (e.g. unlink(), rename(), etc.). Thus, for + * a thread to be doing a "bufwait" -> "dirhash" order, it has to hold + * an exclusive vnode lock. That exclusive vnode lock will prevent + * any other threads from doing a "dirhash" -> "bufwait" order. */ static void
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