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Message-Id: <1254755602.3838.5.camel@mulgrave.site>
Date: Mon, 05 Oct 2009 10:13:22 -0500
From: James Bottomley <James.Bottomley@...e.de>
To: iceberg <strakh@...ras.ru>
Cc: Andrew Morton <akpm@...ux-foundation.org>, eric@...ante.org,
linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
Kay Sievers <kay.sievers@...y.org>, Greg KH <greg@...ah.com>
Subject: Re: [PATCH] scsi_lib.c: sleeping function called from invalid
context
On Mon, 2009-10-05 at 18:35 +0000, iceberg wrote:
> On Thursday 01 October 2009 18:32:16 you wrote:
> > On Thu, 2009-09-24 at 18:23 -0700, James Bottomley wrote:
> > > On Thu, 2009-09-24 at 15:56 -0700, Andrew Morton wrote:
> > > > On Wed, 23 Sep 2009 17:58:47 +0000
> > > >
> > > > iceberg <strakh@...ras.ru> wrote:
> > > > > Driver scsi_lib.c might sleep in atomic context, because it calls
> > > > > scsi_device_put under spin_lock_irqsave.
> > > > > drivers/scsi/scsi_lib.c:356:
> > > > > spin_lock_irqsave(shost->host_lock, flags);
> > > > > scsi_device_put(sdev);
> > > > > Path to might_sleep macro from scsi_device_put:
> > > > > 1. scsi_device_put calls put_device at ./drivers/scsi/scsi.c:1111
> > > > > 2. put_device calls kobject_put at ./drivers/base/core.c:1038
> > > > > 3. kobject_put calls kref_put at ./lib/kobject.c
> > > > > 4. kref_put may call callback function kobject_release at
> > > > > ./lib/kref.c if refcount becomes zero, which might_sleep because it
> > > > > calls user event. Details: 4.1 kobject_cleanup calls kobject_uevent
> > > > > at ./lib/kobject.c:555 4.2 kobject_uevent calls kobject_uevent_env at
> > > > > ./lib/kobject_uevent.c:282 4.3 kobject_uevent_env calls
> > > > > call_usermodehelper_exec at
> > > > > ./include/linux/kmod.h:83
> > > > > 4.4 call_usermodehelper_exec calls wait_for_completion at
> > > > > ./kernel/kmod.c:481
> > > > > 4.5 wait_for_completion calls wait_for_common at
> > > > > ./kernel/sched.c:5710 4.5 wait_for_common calls might_sleep at
> > > > > ./kernels/sched.c:5692
> > > > >
> > > > > Found by Linux Driver Verification project.
> > > > >
> > > > > Delete wrong sleeping function calls.
> > > > >
> > > > > Signed-off-by: Alexander Strakh <strakh@...ras.ru>
> > > > >
> > > > > ---
> > > > > diff --git a/./a/drivers/scsi/scsi_lib.c
> > > > > b/./b/drivers/scsi/scsi_lib.c index f3c4089..a8f8e2f 100644
> > > > > --- a/./a/drivers/scsi/scsi_lib.c
> > > > > +++ b/./b/drivers/scsi/scsi_lib.c
> > > > > @@ -353,9 +353,9 @@ static void scsi_single_lun_run(struct
> > > > > scsi_device *current_sdev)
> > > > >
> > > > > spin_unlock_irqrestore(shost->host_lock, flags);
> > > > > blk_run_queue(sdev->request_queue);
> > > > > - spin_lock_irqsave(shost->host_lock, flags);
> > > > >
> > > > > - scsi_device_put(sdev);
> > > > > + scsi_device_put(sdev);
> > > > > + spin_lock_irqsave(shost->host_lock, flags);
> > > > > }
> > > > > out:
> > > > > spin_unlock_irqrestore(shost->host_lock, flags);
> > > >
> > > > Well this is strange. afacit all the code to which you refer is
> > > > ancient, so why did this bug just pop up now?
> > >
> > > No idea. I think the root cause of this is in the kobject code: we
> > > explicitly require the ability to call last put from interrupt context
> > > (and that includes holding locks). I'll talks to Greg and Kai about
> > > this (they're both here at plumbers). I think the fix is to indirect
> > > the kobject uevent stuff via a usermode helper so we don't get this
> > > problem.
> >
> > Hang on ... I looked at the bug report again: there's no actual kernel
> > trace, just a theoretical function graph.
> >
> > Has this actually been seen or is it just the result of an analysis?
> >
> > If the latter (which I suspect), there's no actual problem. The
> > explicit design of the calls is that device_initialize() and
> > put_device() can be called from interrupt context. device_add() and
> > device_del() must be called from user context.
> >
> > The path you seem to be showing is the put_device() path where there's
> > been an error in the state model and the caller is doing last put on a
> > visible device without having first called device_del().
> >
> > If you see the real kernel message about this, it means there's a bug in
> > the device model handling somewhere in SCSI. If you haven't seen the
> > message, it's just a bug in the static analysis tool.
>
> This bug report is the result of code inspection. I'm considering functions
> which can call might_sleep macro and consequently which can not be called from
> atomic context.
> I choose function scsi_device_put. There are two paths to might_sleep macro.
> First path was shown in the report, second is:
> 1. scsi_device_put calls put_device at ./drivers/scsi/scsi.c:1111
> 2. put_device calls kobject_put at ./drivers/base/core.c:1038
> 3. kobject_put calls kref_put at ./lib/kobject.c
> 4. kref_put may call callback function kobject_release at ./lib/kref.c if
> refcount becomes zero
> 5. kobject_cleanup calls kobject_del at ./lib/kobject.c:562
only if state_in_sysfs is set.
This is only set if the caller previously failed to call kobject_del
(i.e. device_del).
As long as devices follow the proper create->add->del->put paths, the
final put may be called from interrupt context.
Your analysis is wrong because you're basing it on the exception cleanup
paths not the correct calling paths.
James
> 6. kobject_del calls sysfs_remove_dir at ./lib/kobject.c:516
> 7. sysfs_remove_dir calls __sysfs_remove_dir at ./fs/sysfs/dir.c:821
> 8. __sysfs_remove_dir calls sysfs_addrm_start at ./fs/sysfs/dir.c:789
> 9. sysfs_addrm_start calls mutex_lock at ./fs/sysfs/dir.c:377, which
> might_sleep because it calls might_sleep macro.
>
> As you wrote earlier, scsi_device_put was designed with the ability to call
> last put from interrupt context, but as we can see from the paths there might
> be situations where it is not true. Moreover, while analysing different usage
> patterns of scsi_device_put, I found that people are using scsi_device_put as
> if it can not be called from atomic context. Because before calling
> scsi_device_put, spin_locks are always released (i.e. spin_unlock is called
> before scsi_device_put and spin_lock is called after it). Examples are:
> 1. drivers/scsi/dpt_i2o.c line 701
> 2. drivers/ata/libata-scsi.c line 3626
> 3. drivers/scsi/ipr.c line 2415
>
> >The path you seem to be showing is the put_device() path where there's
> >been an error in the state model and the caller is doing last put on a
> >visible device without having first called device_del().
>
> In scsi_lib.c prior to scsi_device_put we always do scsi_device_get. As far
> as I understand, if we are sure that scsi_device_put is always not last, then
> we can remove both calls to scsi_device_get and to scsi_device_put from the
> code without introducing races.
>
> 347 list_for_each_entry_safe(sdev, tmp, &starget->devices,
> 348 same_target_siblings) {
> 349 if (sdev == current_sdev)
> 350 continue;
> 351 if (scsi_device_get(sdev))
> 352 continue;
> 353
> 354 spin_unlock_irqrestore(shost->host_lock, flags);
> 355 blk_run_queue(sdev->request_queue);
> 356 spin_lock_irqsave(shost->host_lock, flags);
> 357
> 358 scsi_device_put(sdev);
> 359 }
>
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