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

On 19.01.2022 01:08, Konstantin Belousov wrote:
> On Wed, Jan 19, 2022 at 01:01:45AM +0300, Vladimir Kondratyev wrote:
>> On 19.01.2022 00:48, Konstantin Belousov wrote:
>>> 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?
>>>
>>
>> dma-buf uses sequence locks for synchronization. If seqlock is taken for
>> write, than thread it holding enters in to critical section to not force
>> readers to spin if writer is preempted. Unfortunately, dma-buf writers
>> execute callbacks which requires locks and spin_lock_irqsave is used for
>> synchronize these callbacks.
> 
> Then, it seems that locking should be changed either to rwlocks or rmlocks,
> not sure which.
> 
> Do you mean our seqlocks as presented in sys/seqc.h, or something Linuxish?

LinuxKPI seqlocks wraps sys/seqc.h

-- 
WBR
Vladimir Kondratyev