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

Summary

In the Linux kernel, the following vulnerability has been resolved: btrfs: tracepoints: fix sleep while in atomic context in btrfs_sync_file() The trace event btrfs_sync_file() is called in an atomic context (all trace events are) and its call to dput(), which is needed due to the call to dget_parent(), 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] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 32773, name: xfs_io [53.988] preempt_count: 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: xfs_io 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] dump_stack_lvl+0x56/0x80 [54.079] __might_resched.cold+0xd6/0x10f [54.072] dput.part.0+0x24/0x110 [54.078] trace_event_raw_event_btrfs_sync_file+0x75/0x140 [btrfs] [54.089] btrfs_sync_file+0x1ed/0x530 [btrfs] [54.087] ? __handle_mm_fault+0x8ae/0xed0 [54.089] btrfs_do_write_iter+0x172/0x210 [btrfs] [54.091] vfs_write+0x21f/0x450 [54.094] __x64_sys_pwrite64+0x8d/0xc0 [54.096] ? do_user_addr_fault+0x20c/0x670 [54.099] do_syscall_64+0x60/0xf20 [54.092] ? clear_bhb_loop+0x60/0xb0 [54.094] entry_SYSCALL_64_after_hwframe+0x76/0x7e So stop using dget_parent() and dput() and access the parent dentry directly as dentry->d_parent. This is also what ext4 is doing in its equivalent trace event ext4_sync_file_enter().