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Subject: Re: [PATCH] aio: Convert ioctx_table to XArray
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        Benjamin LaHaise <bcrl@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        fsdevel <linux-fsdevel@vger.kernel.org>, linux-aio@kvack.org,
        linux-mm <linux-mm@kvack.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Dan Carpenter <dan.carpenter@oracle.com>
References: <20181128183531.5139-1-willy@infradead.org>
 <09e3d156-66fc-ca17-efac-63f080a27a1d@kernel.dk>
 <20181211184553.GH6830@bombadil.infradead.org>
 <75267003-9407-101f-33ee-685e345a2c8a@kernel.dk>
 <20181211185311.GJ6830@bombadil.infradead.org>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <1629a25c-a9e7-e731-230d-e179e9f7a9a7@kernel.dk>
Date:   Tue, 11 Dec 2018 11:54:53 -0700
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On 12/11/18 11:53 AM, Matthew Wilcox wrote:
> On Tue, Dec 11, 2018 at 11:46:53AM -0700, Jens Axboe wrote:
>> On 12/11/18 11:45 AM, Matthew Wilcox wrote:
>>> I think we need the rcu read lock here to prevent ctx from being freed
>>> under us by free_ioctx().
>>
>> Then that begs the question, how about __xa_load() that is already called
>> under RCU read lock?
> 
> I've been considering adding it to the API, yes.  I was under the
> impression that nested rcu_read_lock() calls were not expensive, even
> with CONFIG_PREEMPT.

They are not expensive, but they are not free either. And if we know we
are already under a rcu read lock, it seems pretty pointless. For the
two cases (memremap and aio), the rcu read lock is right there, before
the call. Easy to verify that it's safe.

-- 
Jens Axboe