Re: [Xen-devel] [PATCH] x86: xen: size struct xen_spinlock to always fit in arch_spinlock_t

[David Vrabel <dvrabel@cantab.net>]

On 23/01/2012 19:50, Konrad Rzeszutek Wilk wrote:
> On Mon, Jan 23, 2012 at 07:32:25PM +0000, David Vrabel wrote:
>> From: David Vrabel<david.vrabel@citrix.com>
>>
>> If NR_CPUS<  256 then arch_spinlock_t is only 16 bits wide but struct
>> xen_spinlock is 32 bits.  When a spin lock is contended and
>> xl->spinners is modified the two bytes immediately after the spin lock
>> would be corrupted.
>>
>> This is a regression caused by 84eb950db13ca40a0572ce9957e14723500943d6
>> (x86, ticketlock: Clean up types and accessors) which reduced the size
>> of arch_spinlock_t.
>>
>> Fix this by making xl->spinners a u8 if NR_CPUS<  256.  A
>> BUILD_BUG_ON() is also added to check the sizes of the two structures
>> are compatible.
>>
>> In many cases this was not noticable as there would often be padding
>> bytes after the lock (e.g., if any of CONFIG_GENERIC_LOCKBREAK,
>> CONFIG_DEBUG_SPINLOCK, or CONFIG_DEBUG_LOCK_ALLOC were enabled).
>>
>> The bnx2 driver is affected. In struct bnx2, phy_lock and
>> indirect_lock may have no padding after them.  Contention on phy_lock
>> would corrupt indirect_lock making it appear locked and the driver
>> would deadlock.
>
> Nice find. I think it also affected the ahci driver, and some of the USB
> ones - at least those I saw starting to hang with:

It's possible but keep in mind that this isn't a recent regression.  I 
think it's been around since 3.0 and maybe even earlier.

David