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From: Mel Gorman <mgorman@techsingularity.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Linux-MM <linux-mm@kvack.org>,
	Linux-RT-Users <linux-rt-users@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Chuck Lever <chuck.lever@oracle.com>,
	Jesper Dangaard Brouer <brouer@redhat.com>,
	Matthew Wilcox <willy@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>, Michal Hocko <mhocko@kernel.org>,
	Oscar Salvador <osalvador@suse.de>
Subject: Re: [PATCH 0/11 v2] Use local_lock for pcp protection and reduce stat overhead
Date: Thu, 8 Apr 2021 18:48:04 +0100
Message-ID: <20210408174804.GH3697@techsingularity.net> (raw)
In-Reply-To: <YG7hQeOthUbPBU86@hirez.programming.kicks-ass.net>

On Thu, Apr 08, 2021 at 12:56:01PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 07, 2021 at 09:24:12PM +0100, Mel Gorman wrote:
> > Why local_lock? PREEMPT_RT considers the following sequence to be unsafe
> > as documented in Documentation/locking/locktypes.rst
> > 
> >    local_irq_disable();
> >    raw_spin_lock(&lock);
> 
> Almost, the above is actually OK on RT. The problematic one is:
> 
> 	local_irq_disable();
> 	spin_lock(&lock);
> 
> That doesn't work on RT since spin_lock() turns into a PI-mutex which
> then obviously explodes if it tries to block with IRQs disabled.
> 
> And it so happens, that's exactly the one at hand.

Ok, I completely messed up the leader because it was local_irq_disable()
+ spin_lock() that I was worried about. Once the series is complete,
it is replated with

  local_lock_irq(&lock_lock)
  spin_lock(&lock);

According to Documentation/locking/locktypes.rst, that should be safe.
I'll rephrase the justification.

-- 
Mel Gorman
SUSE Labs

      reply index

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-07 20:24 Mel Gorman
2021-04-07 20:24 ` [PATCH 01/11] mm/page_alloc: Split per cpu page lists and zone stats Mel Gorman
2021-04-12 17:43   ` Vlastimil Babka
2021-04-13 13:27     ` Mel Gorman
2021-04-07 20:24 ` [PATCH 02/11] mm/page_alloc: Convert per-cpu list protection to local_lock Mel Gorman
2021-04-08 10:52   ` Peter Zijlstra
2021-04-08 17:42     ` Mel Gorman
2021-04-09  6:39       ` Peter Zijlstra
2021-04-09  7:59         ` Mel Gorman
2021-04-09  8:24           ` Peter Zijlstra
2021-04-09 13:32             ` Mel Gorman
2021-04-09 18:55               ` Peter Zijlstra
2021-04-12 11:56                 ` Mel Gorman
2021-04-12 21:47                   ` Thomas Gleixner
2021-04-13 16:52                     ` Mel Gorman
2021-04-07 20:24 ` [PATCH 03/11] mm/memory_hotplug: Make unpopulated zones PCP structures unreachable during hot remove Mel Gorman
2021-04-07 20:24 ` [PATCH 04/11] mm/vmstat: Convert NUMA statistics to basic NUMA counters Mel Gorman
2021-04-14 12:56   ` Vlastimil Babka
2021-04-14 15:18     ` Mel Gorman
2021-04-14 15:56       ` Vlastimil Babka
2021-04-15 10:06         ` Mel Gorman
2021-04-07 20:24 ` [PATCH 05/11] mm/vmstat: Inline NUMA event counter updates Mel Gorman
2021-04-07 20:24 ` [PATCH 06/11] mm/page_alloc: Batch the accounting updates in the bulk allocator Mel Gorman
2021-04-07 20:24 ` [PATCH 07/11] mm/page_alloc: Reduce duration that IRQs are disabled for VM counters Mel Gorman
2021-04-07 20:24 ` [PATCH 08/11] mm/page_alloc: Remove duplicate checks if migratetype should be isolated Mel Gorman
2021-04-07 20:24 ` [PATCH 09/11] mm/page_alloc: Explicitly acquire the zone lock in __free_pages_ok Mel Gorman
2021-04-07 20:24 ` [PATCH 10/11] mm/page_alloc: Avoid conflating IRQs disabled with zone->lock Mel Gorman
2021-04-07 20:24 ` [PATCH 11/11] mm/page_alloc: Update PGFREE outside the zone lock in __free_pages_ok Mel Gorman
2021-04-08 10:56 ` [PATCH 0/11 v2] Use local_lock for pcp protection and reduce stat overhead Peter Zijlstra
2021-04-08 17:48   ` Mel Gorman [this message]

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