linux-kernel - Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read

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Date:   Thu, 17 Sep 2020 15:22:30 +0200
From:   peterz@...radead.org
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Oleg Nesterov <oleg@...hat.com>, Boaz Harrosh <boaz@...xistor.com>,
        Hou Tao <houtao1@...wei.com>, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will@...nel.org>, Dennis Zhou <dennis@...nel.org>,
        Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Jan Kara <jack@...e.cz>
Subject: Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for
 read_count

On Thu, Sep 17, 2020 at 01:48:38PM +0100, Matthew Wilcox wrote:
> On Thu, Sep 17, 2020 at 02:01:33PM +0200, Oleg Nesterov wrote:
> > IIUC, file_end_write() was never IRQ safe (at least if !CONFIG_SMP), even
> > before 8129ed2964 ("change sb_writers to use percpu_rw_semaphore"), but this
> > doesn't matter...
> > 
> > Perhaps we can change aio.c, io_uring.c and fs/overlayfs/file.c to avoid
> > file_end_write() in IRQ context, but I am not sure it's worth the trouble.
> 
> If we change bio_endio to invoke the ->bi_end_io callbacks in softirq
> context instead of hardirq context, we can change the pagecache to take

SoftIRQ context has exactly the same problem vs __this_cpu*().