From: Peter Zijlstra <peterz@infradead.org>
To: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: torvalds@linux-foundation.org, paulmck@linux.vnet.ibm.com,
tglx@linutronix.de, akpm@linux-foundation.org, riel@redhat.com,
mgorman@suse.de, oleg@redhat.com, mingo@redhat.com,
minchan@kernel.org, kamezawa.hiroyu@jp.fujitsu.com,
viro@zeniv.linux.org.uk, dave@stgolabs.net,
linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [RFC][PATCH 4/6] SRCU free VMAs
Date: Fri, 24 Oct 2014 09:26:07 +0200 [thread overview]
Message-ID: <20141024072607.GT21513@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <5449C8A6.9080403@cn.fujitsu.com>
On Fri, Oct 24, 2014 at 11:33:58AM +0800, Lai Jiangshan wrote:
> On 10/23/2014 07:03 PM, Peter Zijlstra wrote:
> > On Thu, Oct 23, 2014 at 06:14:45PM +0800, Lai Jiangshan wrote:
> >>
> >>>
> >>> +struct vm_area_struct *find_vma_srcu(struct mm_struct *mm, unsigned long addr)
> >>> +{
> >>> + struct vm_area_struct *vma;
> >>> + unsigned int seq;
> >>> +
> >>> + WARN_ON_ONCE(!srcu_read_lock_held(&vma_srcu));
> >>> +
> >>> + do {
> >>> + seq = read_seqbegin(&mm->mm_seq);
> >>> + vma = __find_vma(mm, addr);
> >>
> >> will the __find_vma() loops for ever due to the rotations in the RBtree?
> >
> > No, a rotation takes a tree and generates a tree, furthermore the
> > rotation has a fairly strict fwd progress guarantee seeing how its now
> > done with preemption disabled.
>
> I can't get the magic.
>
> __find_vma is visiting vma_a,
> vma_a is rotated to near the top due to multiple updates to the mm.
> __find_vma is visiting down to near the bottom, vma_b.
> now vma_b is rotated up to near the top again.
> __find_vma is visiting down to near the bottom, vma_c.
> now vma_c is rotated up to near the top again.
>
> ...
Why would there be that much rotations? Is this a scenario where someone
is endlessly changing the tree?
If you stop updating the tree, the traversal will finish.
This is no different to the reader starvation already present with
seqlocks.
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next prev parent reply other threads:[~2014-10-24 7:26 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-10-20 21:56 [RFC][PATCH 0/6] Another go at speculative page faults Peter Zijlstra
2014-10-20 21:56 ` [RFC][PATCH 1/6] mm: Dont assume page-table invariance during faults Peter Zijlstra
2014-10-20 21:56 ` [RFC][PATCH 2/6] mm: Prepare for FAULT_FLAG_SPECULATIVE Peter Zijlstra
2014-10-20 21:56 ` [RFC][PATCH 3/6] mm: VMA sequence count Peter Zijlstra
2014-10-22 11:26 ` Kirill A. Shutemov
2014-10-22 11:39 ` Peter Zijlstra
2014-10-22 11:53 ` Kirill A. Shutemov
2014-10-22 12:15 ` Peter Zijlstra
2014-10-22 13:44 ` Peter Zijlstra
2014-10-23 12:36 ` Kirill A. Shutemov
2014-10-23 14:22 ` Peter Zijlstra
2014-10-23 15:05 ` Kirill A. Shutemov
2014-10-20 21:56 ` [RFC][PATCH 4/6] SRCU free VMAs Peter Zijlstra
2014-10-20 23:41 ` Linus Torvalds
2014-10-21 8:07 ` Peter Zijlstra
2014-10-24 15:16 ` Christoph Lameter
2014-10-24 15:51 ` Peter Zijlstra
2014-10-24 17:08 ` Christoph Lameter
2014-10-21 8:22 ` Peter Zijlstra
2014-10-23 10:14 ` Lai Jiangshan
2014-10-23 11:03 ` Peter Zijlstra
2014-10-24 3:33 ` Lai Jiangshan
2014-10-24 7:26 ` Peter Zijlstra [this message]
2014-10-20 21:56 ` [RFC][PATCH 5/6] mm: Provide speculative fault infrastructure Peter Zijlstra
2014-10-21 8:35 ` Kirill A. Shutemov
2014-10-21 10:41 ` Peter Zijlstra
2014-10-21 19:00 ` Peter Zijlstra
2014-10-20 21:56 ` [RFC][PATCH 6/6] mm,x86: Add speculative pagefault handling Peter Zijlstra
2014-10-21 0:07 ` [RFC][PATCH 0/6] Another go at speculative page faults Andy Lutomirski
2014-10-21 8:11 ` Peter Zijlstra
2014-10-21 16:23 ` Ingo Molnar
2014-10-21 17:09 ` Kirill A. Shutemov
2014-10-21 17:56 ` Peter Zijlstra
2014-10-23 10:40 ` Lai Jiangshan
2014-10-23 11:04 ` Peter Zijlstra
2014-10-24 7:54 ` Ingo Molnar
2014-10-24 13:14 ` Peter Zijlstra
2014-10-28 5:32 ` Namhyung Kim
2014-10-21 17:25 ` Peter Zijlstra
2014-10-22 12:35 ` Ingo Molnar
2014-10-22 7:34 ` Davidlohr Bueso
2014-10-22 11:29 ` Kirill A. Shutemov
2014-10-22 11:45 ` Peter Zijlstra
2014-10-22 11:55 ` Kirill A. Shutemov
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