CVE-2026-64164: btrfs: tracepoints: fix sleep while in atomic context in btrfs_sync_file()

In the Linux kernel, the following vulnerability has been resolved:

btrfs: tracepoints: fix sleep while in atomic context in btrfssyncfile()

The trace event btrfssyncfile() is called in an atomic context (all trace events are) and its call to dput(), which is needed due to the call to dgetparent(), can sleep, triggering a kernel splat.

This can be reproduced by enabling the trace event and running btrfs/056 from fstests for example. The splat shown in dmesg is the following:

[53.919] BUG: sleeping function called from invalid context at fs/dcache.c:970 [53.947] inatomic(): 1, irqsdisabled(): 0, nonblock: 0, pid: 32773, name: xfsio [53.988] preemptcount: 2, expected: 0 [53.967] RCU nest depth: 0, expected: 0 [53.943] Preemption disabled at: [53.944] [<0000000000000000>] 0x0 [54.078] CPU: 0 UID: 0 PID: 32773 Comm: xfsio Tainted: G W 7.1.0-rc1-btrfs-next-232+ #1 PREEMPT(full) [54.070] Tainted: [W]=WARN [54.071] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.2-0-gea1b7a073390-prebuilt.qemu.org 04/01/2014 [54.072] Call Trace: [54.074] <TASK> [54.076] dumpstacklvl+0x56/0x80 [54.079] mightresched.cold+0xd6/0x10f [54.072] dput.part.0+0x24/0x110 [54.078] traceeventraweventbtrfssyncfile+0x75/0x140 [btrfs] [54.089] btrfssyncfile+0x1ed/0x530 [btrfs] [54.087] ? handlemmfault+0x8ae/0xed0 [54.089] btrfsdowriteiter+0x172/0x210 [btrfs] [54.091] vfswrite+0x21f/0x450 [54.094] x64syspwrite64+0x8d/0xc0 [54.096] ? douseraddrfault+0x20c/0x670 [54.099] dosyscall64+0x60/0xf20 [54.092] ? clearbhbloop+0x60/0xb0 [54.094] entrySYSCALL64afterhwframe+0x76/0x7e

So stop using dgetparent() and dput() and access the parent dentry directly as dentry->dparent. This is also what ext4 is doing in its equivalent trace event ext4syncfileenter().