linux-kernel - Re: [PATCH v4] irqchip: gic: Allow gic_arch

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Date:	Sun, 17 Aug 2014 22:41:23 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Jason Cooper <jason@...edaemon.net>
Cc:	Stephen Boyd <sboyd@...eaurora.org>, linux-arm-msm@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Nicolas Pitre <nico@...aro.org>
Subject: Re: [PATCH v4] irqchip: gic: Allow gic_arch_extn hooks to call
	into scheduler

On Sun, Aug 17, 2014 at 03:04:34PM -0400, Jason Cooper wrote:
> Quoting Nico:
> 
> "Of course it would be good to clarify things wrt Russell's remark
> independently from this patch."
> 
> I took 'independently' to mean "This patch is ok, *and* we need to
> address Russell's concerns in a follow-up patch."
> 
> Nico's Reviewed-by with that comment was sent August 13th.  The most
> recent activity on this thread was also August 13th.  After four days, I
> reasoned there were no objections to his comment.

Right, during the merge window, and during merge windows, I tend to
ignore almost all email now because people don't stop developing, and
they don't take any notice where the mainline cycle is.  In fact, I go
off and do non-kernel work during a merge window and only briefly scan
for bug fixes.

However, I have other concerns with this patch, which I've yet to air.
For example, I don't like this crappy conditional locking that people
keep dreaming up - that kind of stuff makes the kernel much harder to
statically check that everything is correct.  It's an anti-lockdep
strategy.

Secondly, I don't like this:

+       raw_spin_lock(&gic_sgi_lock);
+       /*
+        * Ensure that the gic_cpu_map update above is seen in
+        * gic_raise_softirq() before we redirect any pending SGIs that
+        * may have been raised for the outgoing CPU (cur_cpu_id)
+        */
+       smp_mb__after_unlock_lock();
+       raw_spin_unlock(&gic_sgi_lock);

That goes against the principle of locking, that you lock the data,
not the code.

I have no problem with changing gic_raise_softirq() to use a different
lock, which gic_migrate_target(), and gic_set_affinity() can also use.
There's no need for horrid locking here, because the only thing we're
protecting is gic_map[] and the write to the register to trigger an
IPI - and nothing using gic_arch_extn has any business knowing about
SGIs.

No need for these crappy sgi_map_lock() macros and all the ifdeffery.

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