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.
next prev parent reply other threads:[~2014-10-24 7:29 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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