9p/xen: protect xen_9pfs_front_free against concurrent calls

9p/xen: protect xen_9pfs_front_free against concurrent calls

The xenwatch thread can race with other back-end change notifications
and call xen_9pfs_front_free() twice, hitting the observed general
protection fault due to a double-free. Guard the teardown path so only
one caller can release the front-end state at a time, preventing the
crash.

This is a fix for the following double-free:

[   27.052347] Oops: general protection fault, probably for non-canonical address 0x6b6b6b6b6b6b6b6b: 0000 [#1] SMP DEBUG_PAGEALLOC NOPTI
[   27.052357] CPU: 0 UID: 0 PID: 32 Comm: xenwatch Not tainted 6.18.0-02087-g51ab33fc0a8b-dirty #60 PREEMPT(none)
[   27.052363] RIP: e030:xen_9pfs_front_free+0x1d/0x150
[   27.052368] Code: 90 90 90 90 90 90 90 90 90 90 90 90 90 41 55 41 54 55 48 89 fd 48 c7 c7 48 d0 92 85 53 e8 cb cb 05 00 48 8b 45 08 48 8b 55 00 <48> 3b 28 0f 85 f9 28 35 fe 48 3b 6a 08 0f 85 ef 28 35 fe 48 89 42
[   27.052377] RSP: e02b:ffffc9004016fdd0 EFLAGS: 00010246
[   27.052381] RAX: 6b6b6b6b6b6b6b6b RBX: ffff88800d66e400 RCX: 0000000000000000
[   27.052385] RDX: 6b6b6b6b6b6b6b6b RSI: 0000000000000000 RDI: 0000000000000000
[   27.052389] RBP: ffff88800a887040 R08: 0000000000000000 R09: 0000000000000000
[   27.052393] R10: 0000000000000000 R11: 0000000000000000 R12: ffff888009e46b68
[   27.052397] R13: 0000000000000200 R14: 0000000000000000 R15: ffff88800a887040
[   27.052404] FS:  0000000000000000(0000) GS:ffff88808ca57000(0000) knlGS:0000000000000000
[   27.052408] CS:  e030 DS: 0000 ES: 0000 CR0: 0000000080050033
[   27.052412] CR2: 00007f9714004360 CR3: 0000000004834000 CR4: 0000000000050660
[   27.052418] Call Trace:
[   27.052420]  <TASK>
[   27.052422]  xen_9pfs_front_changed+0x5d5/0x720
[   27.052426]  ? xenbus_otherend_changed+0x72/0x140
[   27.052430]  ? __pfx_xenwatch_thread+0x10/0x10
[   27.052434]  xenwatch_thread+0x94/0x1c0
[   27.052438]  ? __pfx_autoremove_wake_function+0x10/0x10
[   27.052442]  kthread+0xf8/0x240
[   27.052445]  ? __pfx_kthread+0x10/0x10
[   27.052449]  ? __pfx_kthread+0x10/0x10
[   27.052452]  ret_from_fork+0x16b/0x1a0
[   27.052456]  ? __pfx_kthread+0x10/0x10
[   27.052459]  ret_from_fork_asm+0x1a/0x30
[   27.052463]  </TASK>
[   27.052465] Modules linked in:
[   27.052471] ---[ end trace 0000000000000000 ]---

Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
Message-ID: <20260129230348.2390470-1-stefano.stabellini@amd.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>