DEFINE_IDR() and the layer cache

[Nathan Lynch]

Hi Joachim-

Joachim Fenkes wrote:
>
> idr_init(), when called for the first time, sets up the layer
> cache. idr_get_new() and friends expect the cache to exist. Now, what happens
> if the first call to idr_get_new() targets an idr initialized using
> DEFINE_IDR() before idr_init() has been called at least once? Could this
> happen?

unnamed_dev_init() is the first caller of idr_init() and it happens to
be called before the first use of idr_get_new():

[    0.831907]  [<c010615a>] dump_stack+0x16/0x18
[    0.831945]  [<c01c2714>] idr_init+0x3c/0x9c
[    0.831985]  [<c038cab8>] unnamed_dev_init+0xd/0xf
[    0.832027]  [<c037b987>] start_kernel+0x307/0x341
[    0.832068]  [<00000000>] 0x0
[    0.832149]  =======================
[    0.832881] Mount-cache hash table entries: 512
[    0.833432]  [<c010615a>] dump_stack+0x16/0x18
[    0.833470]  [<c01c298f>] idr_get_new_above_int+0x43/0x222
[    0.833541]  [<c01c2b7b>] idr_get_new+0xd/0x28
[    0.833605]  [<c0169243>] set_anon_super+0x36/0xa0
[    0.833667]  [<c01696be>] sget+0x234/0x2c9
[    0.833722]  [<c016a01e>] get_sb_single+0x24/0x8e
[    0.833781]  [<c01a2468>] sysfs_get_sb+0x1c/0x1e
[    0.833867]  [<c0169b63>] vfs_kern_mount+0x41/0x70
[    0.833927]  [<c0169ba8>] kern_mount+0x16/0x18
[    0.833984]  [<c038e494>] sysfs_init+0x57/0xa5
[    0.834026]  [<c038cfe3>] mnt_init+0xc5/0x1cb
[    0.834064]  [<c038cce8>] vfs_caches_init+0x141/0x152
[    0.834104]  [<c037b996>] start_kernel+0x316/0x341
[    0.834144]  [<00000000>] 0x0
[    0.834174]  =======================

It does seem fragile, though.  AFAICT there's no guarantee that a
DEFINE_IDR user couldn't call idr_get_new() before unnamed_dev_init()
runs.

Also, init_id_cache() in idr.c is racy wrt itself.  If the first two
uses of idr_init() occur concurrently, the code could attempt to
create idr_layer_cache twice.