|
31 | 31 | *macros at the bottom of the file. Check if your platform can use |
32 | 32 | *these or needs to override them. |
33 | 33 | * |
34 | | - * Usually, S_LOCK() is implemented in terms ofaneven lower-levelmacro |
35 | | - *TAS(): |
| 34 | + * Usually, S_LOCK() is implemented in terms of even lower-levelmacros |
| 35 | + *TAS() and TAS_SPIN(): |
36 | 36 | * |
37 | 37 | *int TAS(slock_t *lock) |
38 | 38 | *Atomic test-and-set instruction. Attempt to acquire the lock, |
39 | 39 | *but do *not* wait.Returns 0 if successful, nonzero if unable |
40 | 40 | *to acquire the lock. |
41 | 41 | * |
42 | | - *TAS() is NOT part of the API, and should never be called directly. |
| 42 | + *int TAS_SPIN(slock_t *lock) |
| 43 | + *Like TAS(), but this version is used when waiting for a lock |
| 44 | + *previously found to be contended. Typically, this is the |
| 45 | + *same as TAS(), but on some architectures it's better to poll a |
| 46 | + *contended lock using an unlocked instruction and retry the |
| 47 | + *atomic test-and-set only when it appears free. |
43 | 48 | * |
44 | | - *CAUTION: on some platforms TAS() may sometimes report failure to acquire |
45 | | - *a lock even when the lock is not locked. For example, on Alpha TAS() |
46 | | - *will "fail" if interrupted. Therefore TAS() should always be invoked |
47 | | - *in a retry loop, even if you are certain the lock is free. |
| 49 | + *TAS() and TAS_SPIN() are NOT part of the API, and should never be called |
| 50 | + *directly. |
48 | 51 | * |
49 | | - *ANOTHER CAUTION: be sure that TAS() and S_UNLOCK() represent sequence |
50 | | - *points, ie, loads and stores of other values must not be moved across |
51 | | - *a lock or unlock. In most cases it suffices to make the operation be |
52 | | - *done through a "volatile" pointer. |
| 52 | + *CAUTION: on some platforms TAS() and/or TAS_SPIN() may sometimes report |
| 53 | + *failure to acquire a lock even when the lock is not locked. For example, |
| 54 | + *on Alpha TAS() will "fail" if interrupted. Therefore a retry loop must |
| 55 | + *always be used, even if you are certain the lock is free. |
| 56 | + * |
| 57 | + *ANOTHER CAUTION: be sure that TAS(), TAS_SPIN(), and S_UNLOCK() represent |
| 58 | + *sequence points, ie, loads and stores of other values must not be moved |
| 59 | + *across a lock or unlock. In most cases it suffices to make the operation |
| 60 | + *be done through a "volatile" pointer. |
53 | 61 | * |
54 | 62 | *On most supported platforms, TAS() uses a tas() function written |
55 | 63 | *in assembly language to execute a hardware atomic-test-and-set |
@@ -727,6 +735,7 @@ typedef unsigned int slock_t; |
727 | 735 |
|
728 | 736 | #include<ia64/sys/inline.h> |
729 | 737 | #defineTAS(lock) _Asm_xchg(_SZ_W, lock, 1, _LDHINT_NONE) |
| 738 | +#defineTAS_SPIN(lock) (*(lock) ? 1 : TAS(lock)) |
730 | 739 |
|
731 | 740 | #endif/* HPUX on IA64, non gcc */ |
732 | 741 |
|
@@ -925,6 +934,10 @@ extern inttas(volatile slock_t *lock);/* in port/.../tas.s, or |
925 | 934 | #defineTAS(lock)tas(lock) |
926 | 935 | #endif/* TAS */ |
927 | 936 |
|
| 937 | +#if !defined(TAS_SPIN) |
| 938 | +#defineTAS_SPIN(lock)TAS(lock) |
| 939 | +#endif/* TAS_SPIN */ |
| 940 | + |
928 | 941 |
|
929 | 942 | /* |
930 | 943 | * Platform-independent out-of-line support routines |
|