linux-kernel - Re: [PATCH -next] ipc: use GFP

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Message-ID: <20200428111403.GJ29705@bombadil.infradead.org>
Date:   Tue, 28 Apr 2020 04:14:03 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Wei Yongjun <weiyongjun1@...wei.com>
Cc:     Pankaj Bharadiya <pankaj.laxminarayan.bharadiya@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Waiman Long <longman@...hat.com>,
        Manfred Spraul <manfred@...orfullife.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Alexey Dobriyan <adobriyan@...il.com>,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH -next] ipc: use GFP_ATOMIC under spin lock

On Tue, Apr 28, 2020 at 03:47:36AM +0000, Wei Yongjun wrote:
> The function ipc_id_alloc() is called from ipc_addid(), in which
> a spin lock is held, so we should use GFP_ATOMIC instead.
> 
> Fixes: de5738d1c364 ("ipc: convert ipcs_idr to XArray")
> Signed-off-by: Wei Yongjun <weiyongjun1@...wei.com>

I see why you think that, but it's not true.  Yes, we hold a spinlock, but
the spinlock is in an object which is not reachable from any other CPU.
So it's not possible to deadlock.  This probably confuses all kinds
of automated checkers, and I should probably rewrite the code to not
acquire the new spinlock until we're already holding the xa_lock.

Converting to GFP_ATOMIC is completely wrong.