From: David Howells <dhowells@redhat.com>
To: Nick Piggin <npiggin@suse.de>
Cc: dhowells@redhat.com, torvalds@osdl.org,
akpm@linux-foundation.org, paulmck@linux.vnet.ibm.com,
corbet@lwn.net, linux-kernel@vger.kernel.org,
linux-fsdevel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH] radix_tree_tag_get() is not as safe as the docs make out
Date: Tue, 06 Apr 2010 22:18:58 +0100 [thread overview]
Message-ID: <27834.1270588738@redhat.com> (raw)
In-Reply-To: <20100406194843.GJ5288@laptop>
Nick Piggin <npiggin@suse.de> wrote:
> Nack, just drop the BUG_ON.
I can do that.
> I don't know what you mean by "untrustworthy answer".
I was thinking that the answer you get from radix_tree_tag_get() may be invalid
if the tag chain is being modified as you read it. So if you do:
rcu_read_lock()
...
x = radix_tree_tag_get(r, i, t);
...
y = radix_tree_tag_get(r, i, t);
...
rcu_read_unlock()
Then you can't guarantee that x == y, even though you were holding the RCU read
lock.
As you suggested, I'll try and come up with a comment modification to this
effect.
David
WARNING: multiple messages have this Message-ID (diff)
From: David Howells <dhowells@redhat.com>
To: Nick Piggin <npiggin@suse.de>
Cc: dhowells@redhat.com, torvalds@osdl.org,
akpm@linux-foundation.org, paulmck@linux.vnet.ibm.com,
corbet@lwn.net, linux-kernel@vger.kernel.org,
linux-fsdevel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH] radix_tree_tag_get() is not as safe as the docs make out
Date: Tue, 06 Apr 2010 22:18:58 +0100 [thread overview]
Message-ID: <27834.1270588738@redhat.com> (raw)
In-Reply-To: <20100406194843.GJ5288@laptop>
Nick Piggin <npiggin@suse.de> wrote:
> Nack, just drop the BUG_ON.
I can do that.
> I don't know what you mean by "untrustworthy answer".
I was thinking that the answer you get from radix_tree_tag_get() may be invalid
if the tag chain is being modified as you read it. So if you do:
rcu_read_lock()
...
x = radix_tree_tag_get(r, i, t);
...
y = radix_tree_tag_get(r, i, t);
...
rcu_read_unlock()
Then you can't guarantee that x == y, even though you were holding the RCU read
lock.
As you suggested, I'll try and come up with a comment modification to this
effect.
David
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next prev parent reply other threads:[~2010-04-06 21:19 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-04-06 19:31 [PATCH] radix_tree_tag_get() is not as safe as the docs make out David Howells
2010-04-06 19:31 ` David Howells
2010-04-06 19:39 ` Linus Torvalds
2010-04-06 19:39 ` Linus Torvalds
2010-04-06 21:12 ` David Howells
2010-04-06 21:12 ` David Howells
2010-04-06 19:48 ` Nick Piggin
2010-04-06 19:48 ` Nick Piggin
2010-04-06 21:18 ` David Howells [this message]
2010-04-06 21:18 ` David Howells
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