linux-kernel - Re: [PATCH] virtio: console: Unlock vqs while freeing buffers

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Date:   Tue, 25 Oct 2016 16:19:03 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Amit Shah <amit.shah@...hat.com>
Cc:     Matt Redfearn <matt.redfearn@...tec.com>, stable@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] virtio: console: Unlock vqs while freeing buffers

On Tue, Oct 25, 2016 at 12:48:03PM +0530, Amit Shah wrote:
> On (Tue) 11 Oct 2016 [12:05:15], Matt Redfearn wrote:
> > Commit c6017e793b93 ("virtio: console: add locks around buffer removal
> > in port unplug path") added locking around the freeing of buffers in the
> > vq. However, when free_buf() is called with can_sleep = true and rproc
> > is enabled, it calls dma_free_coherent() directly, requiring interrupts
> > to be enabled. Currently a WARNING is triggered due to the spin locking
> > around free_buf, with a call stack like this:
> > 
> > WARNING: CPU: 3 PID: 121 at ./include/linux/dma-mapping.h:433
> > free_buf+0x1a8/0x288
> > Call Trace:
> > [<8040c538>] show_stack+0x74/0xc0
> > [<80757240>] dump_stack+0xd0/0x110
> > [<80430d98>] __warn+0xfc/0x130
> > [<80430ee0>] warn_slowpath_null+0x2c/0x3c
> > [<807e7c6c>] free_buf+0x1a8/0x288
> > [<807ea590>] remove_port_data+0x50/0xac
> > [<807ea6a0>] unplug_port+0xb4/0x1bc
> > [<807ea858>] virtcons_remove+0xb0/0xfc
> > [<807b6734>] virtio_dev_remove+0x58/0xc0
> > [<807f918c>] __device_release_driver+0xac/0x134
> > [<807f924c>] device_release_driver+0x38/0x50
> > [<807f7edc>] bus_remove_device+0xfc/0x130
> > [<807f4b74>] device_del+0x17c/0x21c
> > [<807f4c38>] device_unregister+0x24/0x38
> > [<807b6b50>] unregister_virtio_device+0x28/0x44
> > 
> > Fix this by restructuring the loops to allow the locks to only be taken
> > where it is necessary to protect the vqs, and release it while the
> > buffer is being freed.
> > 
> > Fixes: c6017e793b93 ("virtio: console: add locks around buffer removal in port unplug path")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Matt Redfearn <matt.redfearn@...tec.com>
> 
> Reviewed-by: Amit Shah <amit.shah@...hat.com>
> 
> Michael, can you pick this up?
> 
> Thanks,
> 
> 		Amit

Sure.