Re: getiobuf(x, false) can sleep ?

[Mindaugas Rasiukevicius <rmind%netbsd.org@localhost>]

Manuel Bouyer <bouyer%antioche.eu.org@localhost> wrote:
> > > 
> > > The mutex_enter() in the pool_cache_get_slow()?
> > 
> > This cache won't sleep if PR_NOWAIT, it's interrupt safe.
> > 
> > <...>
> 
> I've been running with the attached patch for a day now (the workaround
> for bp changing under us was already there for a couple days), and the
> "dkstart: preemption occured" printf fired twice. I didn't do the
> kpreempt_disable()/kpreempt_enable() because the code is running
> with the kernel_lock and at splbio(), so kernel preemption should already
> be disabled (if it's not it's a problem).  <...>

This is concerning.  As mentioned, bufio_cache is interrupt safe (marked
with IPL_BIO) and locks at pool_cache(9) layer, also deeper layers e.g.
UVM kva-cache, should use spin-locks at IPL_VM.

There is one possibly related mysterious backtrace - PR/42611.  It looks
like mutex locked against oneself (via interrupt) on interrupt safe pool,
which suggests non-IPL_VM mutex use.  Quick code inspection did not reveal
anything suspicious, but there are many cache layers involved, and I did
not find enough time for a deep look yet.

-- 
Mindaugas