Re: [PATCH] fs/pstore: Perform erase from a worker

[Chris Wilson]

On Tue, Mar 21, 2017 at 02:58:48PM +0900, Namhyung Kim wrote:
> Hello,
> 
> On Mon, Mar 20, 2017 at 10:49:16AM -0700, Kees Cook wrote:
> > On Fri, Mar 17, 2017 at 2:52 AM, Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote:
> > > In order to prevent a cyclic recursion between psi->read_mutex and the
> > > inode_lock, we need to move the pse->erase to a worker.
> > >
> > > [  605.374955] ======================================================
> > > [  605.381281] [ INFO: possible circular locking dependency detected ]
> > > [  605.387679] 4.11.0-rc2-CI-CI_DRM_2352+ #1 Not tainted
> > > [  605.392826] -------------------------------------------------------
> > > [  605.399196] rm/7298 is trying to acquire lock:
> > > [  605.403720]  (&psinfo->read_mutex){+.+.+.}, at: [<ffffffff813e183f>] pstore_unlink+0x3f/0xa0
> > > [  605.412300]
> > > [  605.412300] but task is already holding lock:
> > > [  605.418237]  (&sb->s_type->i_mutex_key#14){++++++}, at: [<ffffffff812157ec>] vfs_unlink+0x4c/0x19
> > > 0
> > > [  605.427397]
> > > [  605.427397] which lock already depends on the new lock.
> > > [  605.427397]
> > > [  605.435770]
> > > [  605.435770] the existing dependency chain (in reverse order) is:
> > > [  605.443396]
> > > [  605.443396] -> #1 (&sb->s_type->i_mutex_key#14){++++++}:
> > > [  605.450347]        lock_acquire+0xc9/0x220
> > > [  605.454551]        down_write+0x3f/0x70
> > > [  605.458484]        pstore_mkfile+0x1f4/0x460
> > > [  605.462835]        pstore_get_records+0x17a/0x320
> > > [  605.467664]        pstore_fill_super+0xa4/0xc0
> > > [  605.472205]        mount_single+0x89/0xb0
> > > [  605.476314]        pstore_mount+0x13/0x20
> > > [  605.480411]        mount_fs+0xf/0x90
> > > [  605.484122]        vfs_kern_mount+0x66/0x170
> > > [  605.488464]        do_mount+0x190/0xd50
> > > [  605.492397]        SyS_mount+0x90/0xd0
> > > [  605.496212]        entry_SYSCALL_64_fastpath+0x1c/0xb1
> > > [  605.501496]
> > > [  605.501496] -> #0 (&psinfo->read_mutex){+.+.+.}:
> > > [  605.507747]        __lock_acquire+0x1ac0/0x1bb0
> > > [  605.512401]        lock_acquire+0xc9/0x220
> > > [  605.516594]        __mutex_lock+0x6e/0x990
> > > [  605.520755]        mutex_lock_nested+0x16/0x20
> > > [  605.525279]        pstore_unlink+0x3f/0xa0
> > > [  605.529465]        vfs_unlink+0xb5/0x190
> > > [  605.533477]        do_unlinkat+0x24c/0x2a0
> > > [  605.537672]        SyS_unlinkat+0x16/0x30
> > > [  605.541781]        entry_SYSCALL_64_fastpath+0x1c/0xb1
> > 
> > If I'm reading this right it's a race between mount and unlink...
> > that's quite a corner case. :)
> > 
> > > [  605.547067]
> > > [  605.547067] other info that might help us debug this:
> > > [  605.547067]
> > > [  605.555221]  Possible unsafe locking scenario:
> > > [  605.555221]
> > > [  605.561280]        CPU0                    CPU1
> > > [  605.565883]        ----                    ----
> > > [  605.570502]   lock(&sb->s_type->i_mutex_key#14);
> > > [  605.575217]                                lock(&psinfo->read_mutex);
> > > [  605.581803]                                lock(&sb->s_type->i_mutex_key#14);
> > > [  605.589159]   lock(&psinfo->read_mutex);
> > 
> > I haven't had time to dig much yet, but I wonder if the locking order
> > on unlink could just be reversed, and the deadlock would go away?
> 
> IIUC, the unlink path locks a file in the root directory, while the
> mount path locks the root directory.  Maybe we can use a subclass?
> (not tested)

More puzzling, or just my confusion, reports from our CI farm say that
this patch breaks removing objects from pstote. :|

Will look forward to better suggestions on how to avoid lockdep.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre