Files
linux/include/uapi/linux
Brendan Jackman 5ffa25502b bpf: Add instructions for atomic_[cmp]xchg
This adds two atomic opcodes, both of which include the BPF_FETCH
flag. XCHG without the BPF_FETCH flag would naturally encode
atomic_set. This is not supported because it would be of limited
value to userspace (it doesn't imply any barriers). CMPXCHG without
BPF_FETCH woulud be an atomic compare-and-write. We don't have such
an operation in the kernel so it isn't provided to BPF either.

There are two significant design decisions made for the CMPXCHG
instruction:

 - To solve the issue that this operation fundamentally has 3
   operands, but we only have two register fields. Therefore the
   operand we compare against (the kernel's API calls it 'old') is
   hard-coded to be R0. x86 has similar design (and A64 doesn't
   have this problem).

   A potential alternative might be to encode the other operand's
   register number in the immediate field.

 - The kernel's atomic_cmpxchg returns the old value, while the C11
   userspace APIs return a boolean indicating the comparison
   result. Which should BPF do? A64 returns the old value. x86 returns
   the old value in the hard-coded register (and also sets a
   flag). That means return-old-value is easier to JIT, so that's
   what we use.

Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210114181751.768687-8-jackmanb@google.com
2021-01-14 18:34:29 -08:00
..
2020-08-22 10:53:12 +01:00
2020-05-28 11:22:14 +02:00
2019-12-18 18:07:31 +01:00
2020-08-12 10:58:00 -07:00
2019-03-07 18:32:01 -08:00
2020-10-09 12:47:02 -06:00
2020-12-08 15:54:16 +01:00
2019-10-09 22:31:14 -04:00
2019-09-25 17:51:39 -07:00
2019-08-02 14:44:02 +10:00
2020-10-01 15:03:40 -04:00
2020-04-27 16:29:41 +05:30
2020-07-19 19:20:22 -07:00
2020-06-24 21:34:11 +02:00
2020-05-14 16:44:25 +02:00
2019-03-27 13:30:07 -07:00
2020-05-28 22:09:47 -04:00
2018-06-18 15:11:53 +10:00
2020-08-27 08:01:47 -07:00
2020-07-19 19:20:22 -07:00
2018-09-05 22:27:11 -07:00
2020-07-13 15:32:56 -07:00
2020-04-20 12:43:24 -07:00
2020-07-24 17:12:41 -07:00
2020-08-27 07:55:59 -07:00
2020-12-09 12:04:01 -07:00
2019-09-08 15:37:04 +02:00
2020-11-26 14:19:10 +01:00
2020-05-21 08:20:35 -06:00
2017-11-28 16:54:00 +01:00
2020-03-29 22:30:57 -07:00
2020-08-18 15:44:44 +02:00
2020-08-27 16:06:47 -04:00
2020-07-09 12:38:41 -07:00
2018-01-14 23:06:30 -05:00
2018-01-16 16:47:29 +01:00
2020-01-18 09:19:18 -05:00
2019-10-02 20:32:27 -06:00
2019-01-22 10:21:45 +01:00
2019-07-30 20:34:34 +02:00
2020-05-21 17:04:07 -07:00
2018-03-20 03:17:41 +02:00
2020-03-29 22:30:57 -07:00
2017-11-16 10:49:00 +09:00
2019-12-18 18:07:31 +01:00
2018-09-03 13:29:38 +02:00
2019-12-09 09:59:07 +01:00
2020-03-29 23:29:08 +02:00
2018-02-06 18:32:44 -08:00
2020-12-03 16:21:08 -07:00
2020-10-23 11:55:28 -04:00
2020-10-23 11:55:28 -04:00
2019-08-01 21:49:46 +02:00
2020-07-13 15:32:56 -07:00
2020-06-24 07:51:01 +02:00