Re: [PATCH] fs: don't needlessly acquire f_lock

[Luis Henriques <luis@igalia.com>]

Hi Christian,

On Fri, Feb 07 2025, Christian Brauner wrote:

> Before 2011 there was no meaningful synchronization between
> read/readdir/write/seek. Only in commit
> ef3d0fd27e90 ("vfs: do (nearly) lockless generic_file_llseek")
> synchronization was added for SEEK_CUR by taking f_lock around
> vfs_setpos().
>
> Then in 2014 full synchronization between read/readdir/write/seek was
> added in commit 9c225f2655e3 ("vfs: atomic f_pos accesses as per POSIX")
> by introducing f_pos_lock for regular files with FMODE_ATOMIC_POS and
> for directories. At that point taking f_lock became unnecessary for such
> files.
>
> So only acquire f_lock for SEEK_CUR if this isn't a file that would have
> acquired f_pos_lock if necessary.

I'm seeing the splat below with current master.  It's unlikely to be
related with this patch, but with recent overlayfs changes.  I'm just
dropping it here before looking, as maybe it has already been reported.

Cheers,
-- 
Luís

[  133.133745] ------------[ cut here ]------------
[  133.133855] WARNING: CPU: 6 PID: 246 at fs/file.c:1201 file_seek_cur_needs_f_lock+0x4a/0x60
[  133.133940] Modules linked in: virtiofs fuse
[  133.134009] CPU: 6 UID: 1000 PID: 246 Comm: ld Not tainted 6.15.0+ #124 PREEMPT(full) 
[  133.134110] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
[  133.134235] RIP: 0010:file_seek_cur_needs_f_lock+0x4a/0x60
[  133.134286] Code: 00 48 ba fe ff ff ff ff ff ff bf 48 83 e8 01 48 39 c2 73 06 b8 01 00 00 00 c3 48 81 c7 90 00 00 00 e8 da 0e db ff 84 c0 75 ea <0f> 0b b8 01 00 00 00 c3 31 c0 c3 66 66 2e 0f 1f 84 00 00 00 00 00
[  133.134471] RSP: 0018:ffffc90000e67ea0 EFLAGS: 00010246
[  133.134526] RAX: 0000000000000000 RBX: fffffffffffffc01 RCX: 7fffffffffffffff
[  133.134683] RDX: bffffffffffffffe RSI: fffffffffffffc01 RDI: ffff888101bd1e90
[  133.135430] RBP: ffff888101bd1e00 R08: 00000000002a3988 R09: 0000000000000000
[  133.136172] R10: ffffc90000e67ed0 R11: 0000000000000000 R12: 7fffffffffffffff
[  133.136351] R13: ffff888101bd1e00 R14: ffff888105d823c0 R15: 0000000000000001
[  133.136433] FS:  00007fd7880d2b28(0000) GS:ffff8884ad411000(0000) knlGS:0000000000000000
[  133.136516] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  133.136586] CR2: 0000559b3af3a520 CR3: 0000000103cb1000 CR4: 0000000000750eb0
[  133.136667] PKRU: 55555554
[  133.136694] Call Trace:
[  133.136720]  <TASK>
[  133.136747]  generic_file_llseek_size+0x93/0x120
[  133.136802]  ovl_llseek+0x86/0xf0
[  133.136844]  ksys_lseek+0x39/0x90
[  133.136884]  do_syscall_64+0x73/0x2c0
[  133.136932]  entry_SYSCALL_64_after_hwframe+0x4b/0x53
[  133.136994] RIP: 0033:0x7fd788098262
[  133.137034] Code: 48 63 d2 48 63 ff 4d 63 c0 b8 09 01 00 00 0f 05 48 89 c7 e8 8a 80 fd ff 48 83 c4 08 c3 48 63 ff 48 63 d2 b8 08 00 00 00 0f 05 <48> 89 c7 e9 70 80 fd ff 8d 47 27 53 89 fb 83 f8 4e 76 27 b8 ec ff
[  133.137223] RSP: 002b:00007fffffaf82c8 EFLAGS: 00000283 ORIG_RAX: 0000000000000008
[  133.137302] RAX: ffffffffffffffda RBX: 00007fd787ba1010 RCX: 00007fd788098262
[  133.137385] RDX: 0000000000000001 RSI: fffffffffffffc01 RDI: 000000000000000f
[  133.137465] RBP: 0000000000000000 R08: 0000000000000064 R09: 00007fd787c3c6a0
[  133.137545] R10: 000000000000000e R11: 0000000000000283 R12: 00007fffffafa694
[  133.137625] R13: 0000000000000039 R14: 0000000000000038 R15: 00007fffffafaa79
[  133.137708]  </TASK>
[  133.137736] irq event stamp: 1034649
[  133.137776] hardirqs last  enabled at (1034657): [<ffffffff8133c642>] __up_console_sem+0x52/0x60
[  133.137872] hardirqs last disabled at (1034664): [<ffffffff8133c627>] __up_console_sem+0x37/0x60
[  133.137966] softirqs last  enabled at (1012640): [<ffffffff812c4884>] irq_exit_rcu+0x74/0x110
[  133.138064] softirqs last disabled at (1012633): [<ffffffff812c4884>] irq_exit_rcu+0x74/0x110
[  133.138161] ---[ end trace 0000000000000000 ]---