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

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Date:   Mon, 04 Mar 2019 12:01:08 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Nicholas Piggin <npiggin@...il.com>, linux-arch@...r.kernel.org,
        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

Nicholas Piggin <npiggin@...il.com> writes:
> Michael Ellerman's on March 3, 2019 7:26 pm:
>> Nicholas Piggin <npiggin@...il.com> writes:
...
>>> 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?

Yeah I thought of that too but it's not great. We'd start semi-randomly
executing the sync in unlock depending on whether someone had done IO on
that CPU prior to the spinlock.

eg.

	writel(x, y);		// sets paca->io_sync
	...	

	<schedule>

	spin_lock(a);
        ...
        // No IO in here
        ...
        spin_unlock(a);		// sync() here because other task did writel().


Which wouldn't be *incorrect*, but would be kind of weird.

cheers