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X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 07 May 2025 14:16:05.3321 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 782e647a-322d-42aa-67f2-08dd8d71b468 X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=3dd8961f-e488-4e60-8e11-a82d994e183d;Ip=[165.204.84.17];Helo=[SATLEXMB03.amd.com] X-MS-Exchange-CrossTenant-AuthSource: SA2PEPF000015C6.namprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: CY8PR12MB8265 On 2025-05-07 05:27, Jürgen Groß wrote: > On 06.05.25 23:09, Jason Andryuk wrote: >> Marek reported seeing a NULL pointer fault in the xenbus_thread >> callstack: >> BUG: kernel NULL pointer dereference, address: 0000000000000000 >> RIP: e030:__wake_up_common+0x4c/0x180 >> Call Trace: >>   >>   __wake_up_common_lock+0x82/0xd0 >>   process_msg+0x18e/0x2f0 >>   xenbus_thread+0x165/0x1c0 >> >> process_msg+0x18e is req->cb(req).  req->cb is set to xs_wake_up(), a >> thin wrapper around wake_up(), or xenbus_dev_queue_reply().  It seems >> like it was xs_wake_up() in this case. >> >> It seems like req may have woken up the xs_wait_for_reply(), which >> kfree()ed the req.  When xenbus_thread resumes, it faults on the zero-ed >> data. >> >> Linux Device Drivers 2nd edition states: >> "Normally, a wake_up call can cause an immediate reschedule to happen, >> meaning that other processes might run before wake_up returns." >> ... which would match the behaviour observed. >> >> Change to keeping two krefs on each request.  One for the caller, and >> one for xenbus_thread.  Each will kref_put() when finished, and the last >> will free it. >> >> This use of kref matches the description in >> Documentation/core-api/kref.rst >> >> Link: https://lore.kernel.org/xen-devel/ZO0WrR5J0xuwDIxW@mail-itl/ >> Reported-by: "Marek Marczykowski-Górecki" >> >> Fixes: fd8aa9095a95 ("xen: optimize xenbus driver for multiple >> concurrent xenstore accesses") >> Cc: stable@vger.kernel.org >> Signed-off-by: Jason Andryuk > > Reviewed-by: Juergen Gross Thanks >> --- >> Kinda RFC-ish as I don't know if it fixes Marek's issue.  This does seem >> like the correct approach if we are seeing req free()ed out from under >> xenbus_thread. > > I think your analysis is correct. When writing this code I didn't think > of wake_up() needing to access req->wq _after_ having woken up the waiter. Yes, this was tricky. One other thing that makes me think this is correct. If this is the same underlying issue: https://lore.kernel.org/xen-devel/Z_lJTyVipJJEpWg2@mail-itl/ The failure is in the unlock: pvqspinlock: lock 0xffff8881029af110 has corrupted value 0x0! WARNING: CPU: 1 PID: 118 at kernel/locking/qspinlock_paravirt.h:504 __pv_queued_spin_unlock_slowpath+0xdc/0x120 Which makes me think the req was fine entering wake_up(), and it's only found to be corrupt on the way out. Regards, Jason