This introduces a "register private expedited" membarrier command which allows eventual removal of important memory barrier constraints on the scheduler fast-paths. It changes how the "private expedited" membarrier command (new to 4.14) is used from user-space. This new command allows processes to register their intent to use the private expedited command. This affects how the expedited private command introduced in 4.14-rc is meant to be used, and should be merged before 4.14 final. Processes are now required to register before using MEMBARRIER_CMD_PRIVATE_EXPEDITED, otherwise that command returns EPERM. This fixes a problem that arose when designing requested extensions to sys_membarrier() to allow JITs to efficiently flush old code from instruction caches. Several potential algorithms are much less painful if the user register intent to use this functionality early on, for example, before the process spawns the second thread. Registering at this time removes the need to interrupt each and every thread in that process at the first expedited sys_membarrier() system call.
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| @@ -52,21 +52,30 @@ · 51 unmodified lines | |||
| 52 | * (non-running threads are de facto in such a | 52 | * (non-running threads are de facto in such a |
| 53 | * state). This only covers threads from the | 53 | * state). This only covers threads from the |
| 54 | * same processes as the caller thread. This | 54 | * same processes as the caller thread. This |
| 55 | * command returns 0. The "expedited" commands | 55 | * command returns 0 on success. The |
| 56 | * complete faster than the non-expedited ones, | 56 | * "expedited" commands complete faster than |
| 57 | * they never block, but have the downside of | 57 | * the non-expedited ones, they never block, |
| 58 | * causing extra overhead. | 58 | * but have the downside of causing extra |
| 59 | * overhead. A process needs to register its | ||
| 60 | * intent to use the private expedited command | ||
| 61 | * prior to using it, otherwise this command | ||
| 62 | * returns -EPERM. | ||
| 63 | * @MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED: | ||
| 64 | * Register the process intent to use | ||
| 65 | * MEMBARRIER_CMD_PRIVATE_EXPEDITED. Always | ||
| 66 | * returns 0. | ||
| 59 | * | 67 | * |
| 60 | * Command to be passed to the membarrier system call. The commands need to | 68 | * Command to be passed to the membarrier system call. The commands need to |
| 61 | * be a single bit each, except for MEMBARRIER_CMD_QUERY which is assigned to | 69 | * be a single bit each, except for MEMBARRIER_CMD_QUERY which is assigned to |
| 62 | * the value 0. | 70 | * the value 0. |
| 63 | */ | 71 | */ |
| 64 | enum membarrier_cmd { | 72 | enum membarrier_cmd { |
| 65 | MEMBARRIER_CMD_QUERY = 0, | 73 | MEMBARRIER_CMD_QUERY = 0, |
| 66 | MEMBARRIER_CMD_SHARED = (1 << 0), | 74 | MEMBARRIER_CMD_SHARED = (1 << 0), |
| 67 | /* reserved for MEMBARRIER_CMD_SHARED_EXPEDITED (1 << 1) */ | 75 | /* reserved for MEMBARRIER_CMD_SHARED_EXPEDITED (1 << 1) */ |
| 68 | /* reserved for MEMBARRIER_CMD_PRIVATE (1 << 2) */ | 76 | /* reserved for MEMBARRIER_CMD_PRIVATE (1 << 2) */ |
| 69 | MEMBARRIER_CMD_PRIVATE_EXPEDITED = (1 << 3), | 77 | MEMBARRIER_CMD_PRIVATE_EXPEDITED = (1 << 3), |
| 78 | MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED = (1 << 4), | ||
| 70 | }; | 79 | }; |
| 71 | 80 | ||
| 72 | #endif /* _UAPI_LINUX_MEMBARRIER_H */ | 81 | #endif /* _UAPI_LINUX_MEMBARRIER_H */ |