Date: Wed, 9 Jun 2010 07:58:37 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-current@freebsd.org, ticso@cicely.de Cc: Bernd Walter <ticso@cicely7.cicely.de> Subject: Re: lock order reversal bufwait/dirhash Message-ID: <201006090758.37330.jhb@freebsd.org> In-Reply-To: <20100609075152.GC72453@cicely7.cicely.de> References: <20100609075152.GC72453@cicely7.cicely.de>
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On Wednesday 09 June 2010 3:51:52 am Bernd Walter wrote: > Got this during installworld (source on NFS, destination UFS on CF-card) > Source is current checked out yesterday. > > lock order reversal: > 1st 0xc28b85b4 bufwait (bufwait) @ /data/builder/c13-2010-06-07/head/sys/kern/vfs_bio.c:2575 > 2nd 0xc343f000 dirhash (dirhash) @ /data/builder/c13-2010-06-07/head/sys/ufs/ufs/ufs_dirhash.c:283 > KDB: stack backtrace: Known false positive. From ufs_dirhash.c: /* * Locking: * * The relationship between inode and dirhash is protected either by an * exclusive vnode lock or the vnode interlock where a shared vnode lock * may be used. The dirhash_mtx is acquired after the dirhash lock. To * handle teardown races, code wishing to lock the dirhash for an inode * when using a shared vnode lock must obtain a private reference on the * dirhash while holding the vnode interlock. They can drop it once they * have obtained the dirhash lock and verified that the dirhash wasn't * recycled while they waited for the dirhash lock. * * ufsdirhash_build() acquires a shared lock on the dirhash when it is * successful. This lock is released after a call to ufsdirhash_lookup(). * * Functions requiring exclusive access use ufsdirhash_acquire() which may * 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. */ -- John Baldwin
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