Re: [3.12-rc] sg_open: leaving the kernel with locks still held!

From: Simon Kirby
Date: Thu Oct 24 2013 - 20:37:22 EST

On Wed, Oct 23, 2013 at 10:10:47AM -0400, Douglas Gilbert wrote:

> On 13-10-23 03:44 AM, James Bottomley wrote:
> >On Tue, 2013-10-22 at 20:41 -0400, Douglas Gilbert wrote:
> >>On 13-10-22 04:56 PM, Simon Kirby wrote:
> >>>Hello!
> >>>
> >>>While trying to figure out why the request queue to sda (ext4) was
> >>>clogging up on one of our btrfs backup boxes, I noticed a megarc process
> >>>in D state, so enabled locking debugging, and got this (on 3.12-rc6):
> >>>
> >>>[ 205.372823] ================================================
> >>>[ 205.372901] [ BUG: lock held when returning to user space! ]
> >>>[ 205.372979] 3.12.0-rc6-hw-debug-pagealloc+ #67 Not tainted
> >>>[ 205.373055] ------------------------------------------------
> >>>[ 205.373132] megarc.bin/5283 is leaving the kernel with locks still held!
> >>>[ 205.373212] 1 lock held by megarc.bin/5283:
> >>>[ 205.373285] #0: (&sdp->o_sem){.+.+..}, at: [<ffffffff8161e650>] sg_open+0x3a0/0x4d0
> >>>
> >>>Vaughan, it seems you touched this area last in 15b06f9a02406e, and git
> >>>tag --contains says this went in for 3.12-rc. We didn't see this on 3.11,
> >>>though I haven't tried with lockdep.
> >>>
> >>>This is caused by some of our internal RAID monitoring scripts that run
> >>>"megarc.bin -dispCfg -a0" (even though that controller isn't present on
> >>>this server -- a PowerEdge 2950 w/Perc 5).
> >>>
> >>>strace output of the program execution that causes the above message is
> >>>here:
> >>
> >>This has been reported. That patch will be reverted or,
> >>if there is enough time, a fix will (or at least should)
> >>go in before the release of lk 3.12 .
> >
> >I think you've got about a week to prove you can fix it (before 3.12
> >goes final). I'll send my current set of fixes to Linus without doing
> >anything about sg.
> "prove" is a big ask, especially coming from a
> mathematician. I consider it more hacking (in the
> golf sense) on my part to tweak well-meaning patches
> to the sg driver that cause collateral damage. Further,
> I suspect Vaughan's patch was an attempt to fix
> damage left be a previous sg_open() hacker.
> I have asked Simon Kirby to apply the patch:
> and report if it fixes his problems. Further I have
> written three test programs to test O_EXCL handling on
> SCSI devices, two of which are in the examples directory
> of sg3_utils version 1.37 . The latest one (single
> exclusive writer, multiple readers) can be found in
> the News section of:
> These tests don't check all possibilities (e.g. random
> signals, ml error processing and detached devices) but
> they are better than nothing. And, as a side issue, they
> break bsg (cause it ignores O_EXCL) and break the block
> layer (e.g. /dev/sdb) so perhaps it should be reverted :-)

Well, this patch works for me in that I see no more lockdep warnings or
unintended consequences when running the same "megarc.bin -dispCfg -a0"

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