1 | /* Elided pthread mutex lock. |
2 | Copyright (C) 2014-2022 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 | <https://www.gnu.org/licenses/>. */ |
18 | |
19 | #include <pthread.h> |
20 | #include <pthreadP.h> |
21 | #include <lowlevellock.h> |
22 | #include <htm.h> |
23 | #include <elision-conf.h> |
24 | #include <stdint.h> |
25 | |
26 | #ifndef EXTRAARG |
27 | #define |
28 | #endif |
29 | #ifndef LLL_LOCK |
30 | #define LLL_LOCK(a,b) lll_lock(a,b), 0 |
31 | #endif |
32 | |
33 | #define aconf __elision_aconf |
34 | |
35 | /* Adaptive lock using transactions. |
36 | By default the lock region is run as a transaction, and when it |
37 | aborts or the lock is busy the lock adapts itself. */ |
38 | |
39 | int |
40 | __lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private) |
41 | { |
42 | /* adapt_count can be accessed concurrently; these accesses can be both |
43 | inside of transactions (if critical sections are nested and the outer |
44 | critical section uses lock elision) and outside of transactions. Thus, |
45 | we need to use atomic accesses to avoid data races. However, the |
46 | value of adapt_count is just a hint, so relaxed MO accesses are |
47 | sufficient. */ |
48 | if (atomic_load_relaxed (adapt_count) <= 0 && aconf.try_tbegin > 0) |
49 | { |
50 | /* Start a transaction and retry it automatically if it aborts with |
51 | _HTM_TBEGIN_TRANSIENT. This macro calls tbegin at most retry_cnt |
52 | + 1 times. The second argument is considered as retry_cnt. */ |
53 | int status = __libc_tbegin_retry ((void *) 0, aconf.try_tbegin - 1); |
54 | if (__glibc_likely (status == _HTM_TBEGIN_STARTED)) |
55 | { |
56 | /* Check the futex to make sure nobody has touched it in the |
57 | mean time. This forces the futex into the cache and makes |
58 | sure the transaction aborts if another thread acquires the lock |
59 | concurrently. */ |
60 | if (__glibc_likely (atomic_load_relaxed (futex) == 0)) |
61 | /* Lock was free. Return to user code in a transaction. */ |
62 | return 0; |
63 | |
64 | /* Lock was busy. Fall back to normal locking. |
65 | This can be the case if e.g. adapt_count was decremented to zero |
66 | by a former release and another thread has been waken up and |
67 | acquired it. */ |
68 | if (__glibc_likely (__libc_tx_nesting_depth () <= 1)) |
69 | { |
70 | /* In a non-nested transaction there is no need to abort, |
71 | which is expensive. Simply end the started transaction. */ |
72 | __libc_tend (); |
73 | /* Don't try to use transactions for the next couple of times. |
74 | See above for why relaxed MO is sufficient. */ |
75 | if (aconf.skip_lock_busy > 0) |
76 | atomic_store_relaxed (adapt_count, aconf.skip_lock_busy); |
77 | } |
78 | else /* nesting depth is > 1 */ |
79 | { |
80 | /* A nested transaction will abort eventually because it |
81 | cannot make any progress before *futex changes back to 0. |
82 | So we may as well abort immediately. |
83 | This persistently aborts the outer transaction to force |
84 | the outer mutex use the default lock instead of retrying |
85 | with transactions until the try_tbegin of the outer mutex |
86 | is zero. |
87 | The adapt_count of this inner mutex is not changed, |
88 | because using the default lock with the inner mutex |
89 | would abort the outer transaction. */ |
90 | __libc_tabort (_HTM_FIRST_USER_ABORT_CODE | 1); |
91 | __builtin_unreachable (); |
92 | } |
93 | } |
94 | else if (status != _HTM_TBEGIN_TRANSIENT) |
95 | { |
96 | /* A persistent abort (cc 1 or 3) indicates that a retry is |
97 | probably futile. Use the normal locking now and for the |
98 | next couple of calls. |
99 | Be careful to avoid writing to the lock. See above for why |
100 | relaxed MO is sufficient. */ |
101 | if (aconf.skip_lock_internal_abort > 0) |
102 | atomic_store_relaxed (adapt_count, |
103 | aconf.skip_lock_internal_abort); |
104 | } |
105 | else |
106 | { |
107 | /* The transaction failed for some retries with |
108 | _HTM_TBEGIN_TRANSIENT. Use the normal locking now and for the |
109 | next couple of calls. */ |
110 | if (aconf.skip_lock_out_of_tbegin_retries > 0) |
111 | atomic_store_relaxed (adapt_count, |
112 | aconf.skip_lock_out_of_tbegin_retries); |
113 | } |
114 | } |
115 | |
116 | /* Use normal locking as fallback path if the transaction does not |
117 | succeed. */ |
118 | return LLL_LOCK ((*futex), private); |
119 | } |
120 | libc_hidden_def (__lll_lock_elision) |
121 | |