1 | /* Elided pthread mutex trylock. |
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 | |
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 elided trylocks. |
34 | Sorry. After the abort the code is re-executed |
35 | non transactional and if the lock was already locked |
36 | return an error. */ |
37 | if (__libc_tx_nesting_depth () > 0) |
38 | { |
39 | /* Note that this abort may terminate an outermost transaction that |
40 | was created outside glibc. |
41 | This persistently aborts the current transactions to force |
42 | them to use the default lock instead of retrying transactions |
43 | until their try_tbegin is zero. |
44 | */ |
45 | __libc_tabort (_HTM_FIRST_USER_ABORT_CODE | 1); |
46 | __builtin_unreachable (); |
47 | } |
48 | |
49 | /* adapt_count can be accessed concurrently; these accesses can be both |
50 | inside of transactions (if critical sections are nested and the outer |
51 | critical section uses lock elision) and outside of transactions. Thus, |
52 | we need to use atomic accesses to avoid data races. However, the |
53 | value of adapt_count is just a hint, so relaxed MO accesses are |
54 | sufficient. */ |
55 | if (atomic_load_relaxed (adapt_count) <= 0 && aconf.try_tbegin > 0) |
56 | { |
57 | int status = __libc_tbegin ((void *) 0); |
58 | if (__glibc_likely (status == _HTM_TBEGIN_STARTED)) |
59 | { |
60 | /* Check the futex to make sure nobody has touched it in the |
61 | mean time. This forces the futex into the cache and makes |
62 | sure the transaction aborts if another thread acquires the lock |
63 | concurrently. */ |
64 | if (__glibc_likely (atomic_load_relaxed (futex) == 0)) |
65 | /* Lock was free. Return to user code in a transaction. */ |
66 | return 0; |
67 | |
68 | /* Lock was busy. Fall back to normal locking. |
69 | This can be the case if e.g. adapt_count was decremented to zero |
70 | by a former release and another thread has been waken up and |
71 | acquired it. |
72 | Since we are in a non-nested transaction there is no need to abort, |
73 | which is expensive. Simply end the started transaction. */ |
74 | __libc_tend (); |
75 | /* Note: Changing the adapt_count here might abort a transaction on a |
76 | different CPU, but that could happen anyway when the futex is |
77 | acquired, so there's no need to check the nesting depth here. |
78 | See above for why relaxed MO is sufficient. */ |
79 | if (aconf.skip_lock_busy > 0) |
80 | atomic_store_relaxed (adapt_count, aconf.skip_lock_busy); |
81 | } |
82 | else if (status != _HTM_TBEGIN_TRANSIENT) |
83 | { |
84 | /* A persistent abort (cc 1 or 3) indicates that a retry is |
85 | probably futile. Use the normal locking now and for the |
86 | next couple of calls. |
87 | Be careful to avoid writing to the lock. */ |
88 | if (aconf.skip_trylock_internal_abort > 0) |
89 | *adapt_count = aconf.skip_trylock_internal_abort; |
90 | } |
91 | /* Could do some retries here. */ |
92 | } |
93 | |
94 | /* Use normal locking as fallback path if the transaction does not |
95 | succeed. */ |
96 | return lll_trylock (*futex); |
97 | } |
98 | libc_hidden_def (__lll_trylock_elision) |
99 | |