Allow expedited membarrier to be used for data shared between processes through shared memory. Processes wishing to receive the membarriers register with MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED. Those which want to issue membarrier invoke MEMBARRIER_CMD_GLOBAL_EXPEDITED. This allows extremely simple kernel-level implementation: we have almost everything we need with the PRIVATE_EXPEDITED barrier code. All we need to do is to add a flag in the mm_struct that will be used to check whether we need to send the IPI to the current thread of each CPU. There is a slight downside to this approach compared to targeting specific shared memory users: when performing a membarrier operation, all registered "global" receivers will get the barrier, even if they don't share a memory mapping with the sender issuing MEMBARRIER_CMD_GLOBAL_EXPEDITED. This registration approach seems to fit the requirement of not disturbing processes that really deeply care about real-time: they simply should not register with MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED. In order to align the membarrier command names, the "MEMBARRIER_CMD_SHARED" command is renamed to "MEMBARRIER_CMD_GLOBAL", keeping an alias of MEMBARRIER_CMD_SHARED to MEMBARRIER_CMD_GLOBAL for UAPI header backward compatibility.
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| @@ -31,7 +31,7 @@ · 30 unmodified lines | ||
| 31 | 31 | * enum membarrier_cmd - membarrier system call command |
| 32 | 32 | * @MEMBARRIER_CMD_QUERY: Query the set of supported commands. It returns |
| 33 | 33 | * a bitmask of valid commands. |
| 34 | − * @MEMBARRIER_CMD_SHARED: Execute a memory barrier on all running threads. | |
| 34 | + * @MEMBARRIER_CMD_GLOBAL: Execute a memory barrier on all running threads. | |
| 35 | 35 | * Upon return from system call, the caller thread |
| 36 | 36 | * is ensured that all running threads have passed |
| 37 | 37 | * through a state where all memory accesses to |
| @@ -40,6 +40,28 @@ · 2 unmodified lines | ||
| 40 | 40 | * (non-running threads are de facto in such a |
| 41 | 41 | * state). This covers threads from all processes |
| 42 | 42 | * running on the system. This command returns 0. |
| 43 | + * @MEMBARRIER_CMD_GLOBAL_EXPEDITED: | |
| 44 | + * Execute a memory barrier on all running threads | |
| 45 | + * of all processes which previously registered | |
| 46 | + * with MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED. | |
| 47 | + * Upon return from system call, the caller thread | |
| 48 | + * is ensured that all running threads have passed | |
| 49 | + * through a state where all memory accesses to | |
| 50 | + * user-space addresses match program order between | |
| 51 | + * entry to and return from the system call | |
| 52 | + * (non-running threads are de facto in such a | |
| 53 | + * state). This only covers threads from processes | |
| 54 | + * which registered with | |
| 55 | + * MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED. | |
| 56 | + * This command returns 0. Given that | |
| 57 | + * registration is about the intent to receive | |
| 58 | + * the barriers, it is valid to invoke | |
| 59 | + * MEMBARRIER_CMD_GLOBAL_EXPEDITED from a | |
| 60 | + * non-registered process. | |
| 61 | + * @MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED: | |
| 62 | + * Register the process intent to receive | |
| 63 | + * MEMBARRIER_CMD_GLOBAL_EXPEDITED memory | |
| 64 | + * barriers. Always returns 0. | |
| 43 | 65 | * @MEMBARRIER_CMD_PRIVATE_EXPEDITED: |
| 44 | 66 | * Execute a memory barrier on each running |
| 45 | 67 | * thread belonging to the same process as the current |
| @@ -64,18 +86,24 @@ · 18 unmodified lines | ||
| 64 | 86 | * Register the process intent to use |
| 65 | 87 | * MEMBARRIER_CMD_PRIVATE_EXPEDITED. Always |
| 66 | 88 | * returns 0. |
| 89 | + * @MEMBARRIER_CMD_SHARED: | |
| 90 | + * Alias to MEMBARRIER_CMD_GLOBAL. Provided for | |
| 91 | + * header backward compatibility. | |
| 67 | 92 | * |
| 68 | 93 | * Command to be passed to the membarrier system call. The commands need to |
| 69 | 94 | * be a single bit each, except for MEMBARRIER_CMD_QUERY which is assigned to |
| 70 | 95 | * the value 0. |
| 71 | 96 | */ |
| 72 | 97 | enum membarrier_cmd { |
| 73 | − MEMBARRIER_CMD_QUERY = 0, | |
| 74 | − MEMBARRIER_CMD_SHARED = (1 << 0), | |
| 75 | − /* reserved for MEMBARRIER_CMD_SHARED_EXPEDITED (1 << 1) */ | |
| 76 | − /* reserved for MEMBARRIER_CMD_PRIVATE (1 << 2) */ | |
| 77 | − MEMBARRIER_CMD_PRIVATE_EXPEDITED = (1 << 3), | |
| 78 | − MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED = (1 << 4), | |
| 98 | + MEMBARRIER_CMD_QUERY = 0, | |
| 99 | + MEMBARRIER_CMD_GLOBAL = (1 << 0), | |
| 100 | + MEMBARRIER_CMD_GLOBAL_EXPEDITED = (1 << 1), | |
| 101 | + MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED = (1 << 2), | |
| 102 | + MEMBARRIER_CMD_PRIVATE_EXPEDITED = (1 << 3), | |
| 103 | + MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED = (1 << 4), | |
| 104 | + | |
| 105 | + /* Alias for header backward compatibility. */ | |
| 106 | + MEMBARRIER_CMD_SHARED = MEMBARRIER_CMD_GLOBAL, | |
| 79 | 107 | }; |
| 80 | 108 | |
| 81 | 109 | #endif /* _UAPI_LINUX_MEMBARRIER_H */ |