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Date:   Tue, 8 Jun 2021 17:30:44 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     chenguanyou <chenguanyou9338@...il.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        chenguanyou <chenguanyou@...omi.com>
Subject: Re: [PATCH] [fuse] alloc_page nofs avoid deadlock

On Thu, 3 Jun 2021 at 14:52, chenguanyou <chenguanyou9338@...il.com> wrote:
>
> ABA deadlock
>
> PID: 17172 TASK: ffffffc0c162c000 CPU: 6 COMMAND: "Thread-21"
> 0 [ffffff802d16b400] __switch_to at ffffff8008086a4c
> 1 [ffffff802d16b470] __schedule at ffffff80091ffe58
> 2 [ffffff802d16b4d0] schedule at ffffff8009200348
> 3 [ffffff802d16b4f0] bit_wait at ffffff8009201098
> 4 [ffffff802d16b510] __wait_on_bit at ffffff8009200a34
> 5 [ffffff802d16b5b0] inode_wait_for_writeback at ffffff800830e1e8
> 6 [ffffff802d16b5e0] evict at ffffff80082fb15c
> 7 [ffffff802d16b620] iput at ffffff80082f9270
> 8 [ffffff802d16b680] dentry_unlink_inode at ffffff80082f4c90
> 9 [ffffff802d16b6a0] __dentry_kill at ffffff80082f1710
> 10 [ffffff802d16b6d0] shrink_dentry_list at ffffff80082f1c34
> 11 [ffffff802d16b750] prune_dcache_sb at ffffff80082f18a8
> 12 [ffffff802d16b770] super_cache_scan at ffffff80082d55ac
> 13 [ffffff802d16b860] shrink_slab at ffffff8008266170
> 14 [ffffff802d16b900] shrink_node at ffffff800826b420
> 15 [ffffff802d16b980] do_try_to_free_pages at ffffff8008268460
> 16 [ffffff802d16ba60] try_to_free_pages at ffffff80082680d0
> 17 [ffffff802d16bbe0] __alloc_pages_nodemask at ffffff8008256514
> 18 [ffffff802d16bc60] fuse_copy_fill at ffffff8008438268
> 19 [ffffff802d16bd00] fuse_dev_do_read at ffffff8008437654
> 20 [ffffff802d16bdc0] fuse_dev_splice_read at ffffff8008436f40
> 21 [ffffff802d16be60] sys_splice at ffffff8008315d18
> 22 [ffffff802d16bff0] __sys_trace at ffffff8008084014
>
> PID: 9652 TASK: ffffffc0c9ce0000 CPU: 4 COMMAND: "kworker/u16:8"
> 0 [ffffff802e793650] __switch_to at ffffff8008086a4c
> 1 [ffffff802e7936c0] __schedule at ffffff80091ffe58
> 2 [ffffff802e793720] schedule at ffffff8009200348
> 3 [ffffff802e793770] __fuse_request_send at ffffff8008435760
> 4 [ffffff802e7937b0] fuse_simple_request at ffffff8008435b14
> 5 [ffffff802e793930] fuse_flush_times at ffffff800843a7a0
> 6 [ffffff802e793950] fuse_write_inode at ffffff800843e4dc
> 7 [ffffff802e793980] __writeback_single_inode at ffffff8008312740
> 8 [ffffff802e793aa0] writeback_sb_inodes at ffffff80083117e4
> 9 [ffffff802e793b00] __writeback_inodes_wb at ffffff8008311d98
> 10 [ffffff802e793c00] wb_writeback at ffffff8008310cfc
> 11 [ffffff802e793d00] wb_workfn at ffffff800830e4a8
> 12 [ffffff802e793d90] process_one_work at ffffff80080e4fac
> 13 [ffffff802e793e00] worker_thread at ffffff80080e5670
> 14 [ffffff802e793e60] kthread at ffffff80080eb650

The issue is real.

The fix, however, is not the right one.  The fundamental problem is
that fuse_write_inode() blocks on a request to userspace.

This is the same issue that fuse_writepage/fuse_writepages face.  In
that case the solution was to copy the page contents to a temporary
buffer and return immediately as if the writeback already completed.

Something similar needs to be done here: send the FUSE_SETATTR request
asynchronously and return immediately from fuse_write_inode().  The
tricky part is to make sure that multiple time updates for the same
inode aren't mixed up...

Thanks,
Miklos

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