linux-kernel - Re: [PATCH 34/33] netfs: Use in_interrupt() not in

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Date:   Tue, 16 Feb 2021 09:29:31 +0000
From:   David Howells <dhowells@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     dhowells@...hat.com, Trond Myklebust <trondmy@...merspace.com>,
        Marc Dionne <marc.dionne@...istor.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        linux-cifs@...r.kernel.org, ceph-devel@...r.kernel.org,
        Jeff Layton <jlayton@...hat.com>,
        Matthew Wilcox <willy@...radead.org>, linux-cachefs@...hat.com,
        Alexander Viro <viro@...iv.linux.org.uk>, linux-mm@...ck.org,
        linux-afs@...ts.infradead.org,
        v9fs-developer@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org,
        Jeff Layton <jlayton@...nel.org>,
        David Wysochanski <dwysocha@...hat.com>,
        linux-kernel@...r.kernel.org,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH 34/33] netfs: Use in_interrupt() not in_softirq()

Christoph Hellwig <hch@....de> wrote:

> On Mon, Feb 15, 2021 at 10:46:23PM +0000, David Howells wrote:
> > The in_softirq() in netfs_rreq_terminated() works fine for the cache being
> > on a normal disk, as the completion handlers may get called in softirq
> > context, but for an NVMe drive, the completion handler may get called in
> > IRQ context.
> > 
> > Fix to use in_interrupt() instead of in_softirq() throughout the read
> > helpers, particularly when deciding whether to punt code that might sleep
> > off to a worker thread.
> 
> We must not use either check, as they all are unreliable especially
> for PREEMPT-RT.

Is there a better way to do it?  The intent is to process the assessment phase
in the calling thread's context if possible rather than bumping over to a
worker thread.  For synchronous I/O, for example, that's done in the caller's
thread.  Maybe that's the answer - if it's known to be asynchronous, I have to
punt, but otherwise don't have to.

David