Date: Wed, 16 Mar 2016 15:12:20 -0700 From: John Baldwin <jhb@freebsd.org> To: Bryan Drewery <bdrewery@freebsd.org> Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r296947 - head/share/man/man9 Message-ID: <1525892.cfDzejblxM@ralph.baldwin.cx> In-Reply-To: <201603161839.u2GIdm5C072960@repo.freebsd.org> References: <201603161839.u2GIdm5C072960@repo.freebsd.org>
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On Wednesday, March 16, 2016 06:39:48 PM Bryan Drewery wrote: > Author: bdrewery > Date: Wed Mar 16 18:39:48 2016 > New Revision: 296947 > URL: https://svnweb.freebsd.org/changeset/base/296947 > > Log: > Remove incorrect BUGS entry about asserting lock not held. > > For non-WITNESS< assertion support for SA_UNLOCKED was added in r125421 and > made to panic in r126316. > > MFC after: 1 week Eh, how can this possibly work? That is, suppose I have this code: sx_slock(&foo); sx_assert(&foo, SA_UNLOCKED); How does that safely work without WITNESS? It needs to not panic in the case that some other thread does sx_slock(&foo). In fact, the comment (modulo the spelling nit) says this explicitly: case SA_UNLOCKED: #ifdef WITNESS witness_assert(&sx->lock_object, what, file, line); #else /* * If we hold an exclusve lock fail. We can't * reliably check to see if we hold a shared lock or * not. */ if (sx_xholder(sx) == curthread) panic("Lock %s exclusively locked @ %s:%d\n", sx->lock_object.lo_name, file, line); #endif break; You could perhaps reword the bug to say that SA_UNLOCKED will panic if the lock is exclusively locked in the non-WITNESS case, but will fail to panic if the thread holds a shared lock. The wording in rwlock(9) is better than sx(9) though it should mention that RA_UNLOCKED can detect a write lock. That wording could then be used in sx(9). -- John Baldwin
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