1 | /* elision-trylock.c: Lock eliding trylock for pthreads. |
2 | Copyright (C) 2013-2016 Free Software Foundation, Inc. |
3 | This file is part of the GNU C Library. |
4 | |
5 | The GNU C Library is free software; you can redistribute it and/or |
6 | modify it under the terms of the GNU Lesser General Public |
7 | License as published by the Free Software Foundation; either |
8 | version 2.1 of the License, or (at your option) any later version. |
9 | |
10 | The GNU C Library is distributed in the hope that it will be useful, |
11 | but WITHOUT ANY WARRANTY; without even the implied warranty of |
12 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
13 | Lesser General Public License for more details. |
14 | |
15 | You should have received a copy of the GNU Lesser General Public |
16 | License along with the GNU C Library; if not, see |
17 | <http://www.gnu.org/licenses/>. */ |
18 | |
19 | #include <pthread.h> |
20 | #include <pthreadP.h> |
21 | #include <lowlevellock.h> |
22 | #include "hle.h" |
23 | #include <elision-conf.h> |
24 | |
25 | #define aconf __elision_aconf |
26 | |
27 | /* Try to elide a futex trylock. FUTEX is the futex variable. ADAPT_COUNT is |
28 | the adaptation counter in the mutex. */ |
29 | |
30 | int |
31 | __lll_trylock_elision (int *futex, short *adapt_count) |
32 | { |
33 | /* Implement POSIX semantics by forbiding nesting |
34 | trylock. Sorry. After the abort the code is re-executed |
35 | non transactional and if the lock was already locked |
36 | return an error. */ |
37 | _xabort (_ABORT_NESTED_TRYLOCK); |
38 | |
39 | /* Only try a transaction if it's worth it. */ |
40 | if (*adapt_count <= 0) |
41 | { |
42 | unsigned status; |
43 | |
44 | if ((status = _xbegin()) == _XBEGIN_STARTED) |
45 | { |
46 | if (*futex == 0) |
47 | return 0; |
48 | |
49 | /* Lock was busy. Fall back to normal locking. |
50 | Could also _xend here but xabort with 0xff code |
51 | is more visible in the profiler. */ |
52 | _xabort (_ABORT_LOCK_BUSY); |
53 | } |
54 | |
55 | if (!(status & _XABORT_RETRY)) |
56 | { |
57 | /* Internal abort. No chance for retry. For future |
58 | locks don't try speculation for some time. */ |
59 | if (*adapt_count != aconf.skip_trylock_internal_abort) |
60 | *adapt_count = aconf.skip_trylock_internal_abort; |
61 | } |
62 | /* Could do some retries here. */ |
63 | } |
64 | else |
65 | { |
66 | /* Lost updates are possible, but harmless. */ |
67 | (*adapt_count)--; |
68 | } |
69 | |
70 | return lll_trylock (*futex); |
71 | } |
72 | |