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Message-ID: <1425922630.2475.390.camel@j-VirtualBox>
Subject: Re: [PATCH] locking/rwsem: Fix lock optimistic spinning when owner
 is not running
From: Jason Low <jason.low2@hp.com>
To: Sasha Levin <sasha.levin@oracle.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, Peter Zijlstra <peterz@infradead.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Michel Lespinasse <walken@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Dave Jones <davej@codemonkey.org.uk>,
        Ming Lei <ming.lei@canonical.com>, jason.low2@hp.com
Date: Mon, 09 Mar 2015 10:37:10 -0700
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On Sat, 2015-03-07 at 13:17 -0500, Sasha Levin wrote:
> On 03/07/2015 02:45 AM, Jason Low wrote:
> > Fixes tip commit b3fd4f03ca0b (locking/rwsem: Avoid deceiving lock spinners).
> > 
> > Ming reported soft lockups occurring when running xfstest due to
> > commit b3fd4f03ca0b.
> > 
> > When doing optimistic spinning in rwsem, threads should stop spinning when
> > the lock owner is not running. While a thread is spinning on owner, if
> > the owner reschedules, owner->on_cpu returns false and we stop spinning.
> > 
> > However, commit b3fd4f03ca0b essentially caused the check to get ignored
> > because when we break out of the spin loop due to !on_cpu, we continue
> > spinning if sem->owner != NULL.
> > 
> > This patch fixes this by making sure we stop spinning if the owner is not
> > running. Furthermore, just like with mutexes, refactor the code such that
> > we don't have separate checks for owner_running(). This makes it more
> > straightforward in terms of why we exit the spin on owner loop and we
> > would also avoid needing to "guess" why we broke out of the loop to make
> > this more readable.
> 
> That seems to solve the hangs I'm seeing as well.

Great, thanks for confirming this.