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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