linux-kernel - Re: [PATCH 01/20] asm-generic/mmiowb: Add generic implementation of mmiowb() tracking

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Date:   Sun, 03 Mar 2019 20:07:52 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     linux-arch@...r.kernel.org, Michael Ellerman <mpe@...erman.id.au>,
        Will Deacon <will.deacon@....com>
Cc:     Andrea Parri <andrea.parri@...rulasolutions.com>,
        Arnd Bergmann <arnd@...db.de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Rich Felker <dalias@...c.org>,
        David Howells <dhowells@...hat.com>,
        Daniel Lustig <dlustig@...dia.com>,
        linux-kernel@...r.kernel.org,
        "Maciej W. Rozycki" <macro@...ux-mips.org>,
        Ingo Molnar <mingo@...nel.org>,
        Palmer Dabbelt <palmer@...ive.com>,
        Paul Burton <paul.burton@...s.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Tony Luck <tony.luck@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>
Subject: Re: [PATCH 01/20] asm-generic/mmiowb: Add generic implementation of
 mmiowb() tracking

Michael Ellerman's on March 3, 2019 7:26 pm:
> Nicholas Piggin <npiggin@...il.com> writes:
>> Will Deacon's on March 2, 2019 12:03 am:
>>> In preparation for removing all explicit mmiowb() calls from driver
>>> code, implement a tracking system in asm-generic based loosely on the
>>> PowerPC implementation. This allows architectures with a non-empty
>>> mmiowb() definition to have the barrier automatically inserted in
>>> spin_unlock() following a critical section containing an I/O write.
>>
>> Is there a reason to call this "mmiowb"? We already have wmb that
>> orders cacheable stores vs mmio stores don't we?
>>
>> Yes ia64 "sn2" is broken in that case, but that can be fixed (if
>> anyone really cares about the platform any more). Maybe that's
>> orthogonal to what you're doing here, I just don't like seeing
>> "mmiowb" spread.
>>
>> This series works for spin locks, but you would want a driver to
>> be able to use wmb() to order locks vs mmio when using a bit lock
>> or a mutex or whatever else. Calling your wmb-if-io-is-pending
>> version io_mb_before_unlock() would kind of match with existing
>> patterns.
>>
>>> +static inline void mmiowb_set_pending(void)
>>> +{
>>> +	struct mmiowb_state *ms = __mmiowb_state();
>>> +	ms->mmiowb_pending = ms->nesting_count;
>>> +}
>>> +
>>> +static inline void mmiowb_spin_lock(void)
>>> +{
>>> +	struct mmiowb_state *ms = __mmiowb_state();
>>> +	ms->nesting_count++;
>>> +}
>>> +
>>> +static inline void mmiowb_spin_unlock(void)
>>> +{
>>> +	struct mmiowb_state *ms = __mmiowb_state();
>>> +
>>> +	if (unlikely(ms->mmiowb_pending)) {
>>> +		ms->mmiowb_pending = 0;
>>> +		mmiowb();
>>> +	}
>>> +
>>> +	ms->nesting_count--;
>>> +}
>>
>> Humour me for a minute and tell me what this algorithm is doing, or
>> what was broken about the powerpc one, which is basically:
>>
>> static inline void mmiowb_set_pending(void)
>> {
>> 	struct mmiowb_state *ms = __mmiowb_state();
>> 	ms->mmiowb_pending = 1;
>> }
>>
>> static inline void mmiowb_spin_lock(void)
>> {
>> }
> 
> The current powerpc code clears io_sync in spin_lock().
> 
> ie, it would be equivalent to:
> 
> static inline void mmiowb_spin_lock(void)
> {
>  	ms->mmiowb_pending = 0;
> }

Ah okay that's what I missed. How about we just not do that?

Thanks,
Nick