Re: git: 02ea6033020e - main - LinuxKPI: Allow spin_lock_irqsave to be called within a critical section

On Wed, Jan 19, 2022 at 12:35:41AM +0300, Vladimir Kondratyev wrote:
> On 18.01.2022 23:22, Konstantin Belousov wrote:
> > On Tue, Jan 18, 2022 at 08:15:36PM +0000, Vladimir Kondratyev wrote:
> > > The branch main has been updated by wulf:
> > > 
> > > URL: https://cgit.FreeBSD.org/src/commit/?id=02ea6033020e11afec6472bf560b0ddebd0fa97a
> > > 
> > > commit 02ea6033020e11afec6472bf560b0ddebd0fa97a
> > > Author:     Vladimir Kondratyev <wulf@FreeBSD.org>
> > > AuthorDate: 2022-01-18 20:14:12 +0000
> > > Commit:     Vladimir Kondratyev <wulf@FreeBSD.org>
> > > CommitDate: 2022-01-18 20:14:12 +0000
> > > 
> > >      LinuxKPI: Allow spin_lock_irqsave to be called within a critical section
> > >      with spinning on spin_trylock. dma-buf part of drm-kmod depends on this
> > >      property and absence of it support results in "mi_switch: switch in a
> > >      critical section" assertions [1][2].
> > >      [1] https://github.com/freebsd/drm-kmod/issues/116
> > >      [2] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=261166
> > >      MFC after:      1 week
> > >      Reviewed by:    manu
> > >      Differential Revision:  https://reviews.freebsd.org/D33887
> > > ---
> > >   .../linuxkpi/common/include/linux/spinlock.h       | 27 ++++++++++++++++++----
> > >   1 file changed, 23 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/sys/compat/linuxkpi/common/include/linux/spinlock.h b/sys/compat/linuxkpi/common/include/linux/spinlock.h
> > > index a87cb7180b28..31d47fa73986 100644
> > > --- a/sys/compat/linuxkpi/common/include/linux/spinlock.h
> > > +++ b/sys/compat/linuxkpi/common/include/linux/spinlock.h
> > > @@ -37,6 +37,7 @@
> > >   #include <sys/lock.h>
> > >   #include <sys/mutex.h>
> > >   #include <sys/kdb.h>
> > > +#include <sys/proc.h>
> > >   #include <linux/compiler.h>
> > >   #include <linux/rwlock.h>
> > > @@ -117,14 +118,32 @@ typedef struct {
> > >   	local_bh_disable();			\
> > >   } while (0)
> > > -#define	spin_lock_irqsave(_l, flags) do {	\
> > > -	(flags) = 0;				\
> > > -	spin_lock(_l);				\
> > > +#define	__spin_trylock_nested(_l, _n) ({		\
> > > +	int __ret;					\
> > > +	if (SPIN_SKIP()) {				\
> > > +		__ret = 1;				\
> > > +	} else {					\
> > > +		__ret = mtx_trylock_flags(&(_l)->m, MTX_DUPOK);	\
> > > +		if (likely(__ret != 0))			\
> > > +			local_bh_disable();		\
> > > +	}						\
> > > +	__ret;						\
> > > +})
> > > +
> > > +#define	spin_lock_irqsave(_l, flags) do {		\
> > > +	(flags) = 0;					\
> > > +	if (unlikely(curthread->td_critnest != 0))	\
> > > +		while (!spin_trylock(_l)) {}		\
> > > +	else						\
> > > +		spin_lock(_l);				\
> > >   } while (0)
> > >   #define	spin_lock_irqsave_nested(_l, flags, _n) do {	\
> > >   	(flags) = 0;					\
> > > -	spin_lock_nested(_l, _n);			\
> > > +	if (unlikely(curthread->td_critnest != 0))	\
> > > +		while (!__spin_trylock_nested(_l, _n)) {}	\
> > > +	else						\
> > > +		spin_lock_nested(_l, _n);		\
> > >   } while (0)
> > >   #define	spin_unlock_irqrestore(_l, flags) do {		\
> > You are spin-waiting for blockable mutex, am I right?
> 
> Both, yes and no. On Linux spin_lock_irqsave is generally unblockable as it
> disables preemption and interrupts while our version does not do this as
> LinuxKPI is not ready for such a tricks.
> It seems that we should explicitly add critical_enter()/critical_exit calls
> to related dma-buf parts to make it unblockable too.
LinuxKPI does +1 to the level of locks comparing with Linux, so their spinlocks
become our blockable mutexes.

Can you please explain why dmabufs need critical section? What is
achieved there by disabled preemption?

> 
>   This means a deadlock.
> > Just for example, on UP machine the spinning thread could starve the thread
> > that owns the mutex, which never gets to the CPU.
> 
> Any ideas how to avoid it in a generic way?
First I need to understand why do you need to mix mutexes and critical
sections.