diff options
Diffstat (limited to 'lib/thread_finish.asm')
| -rw-r--r-- | lib/thread_finish.asm | 48 |
1 files changed, 27 insertions, 21 deletions
diff --git a/lib/thread_finish.asm b/lib/thread_finish.asm index 9392f1c..860b0a4 100644 --- a/lib/thread_finish.asm +++ b/lib/thread_finish.asm @@ -3,9 +3,9 @@ ; Cheat sheet for Linux' x86_64 calling convention: ; -; - free to overwrite; the caller should save them: +; - free to overwrite (caller should save them): ; rax, rcx, rdx, rsi, rdi, r8-r11, xmm0-xmm15 -; - caller expects no change; callee should save them: +; - caller expects be kept (callee should save them): ; rbx, rbp, r12-r15 ; ; - for passing paramters to functions: @@ -28,8 +28,8 @@ section .text %define SYS_MUNMAP 11 %define SYS_FUTEX 202 -; Relevant flags for futex -%define FUTEX_WAIT 0x00 +; Relevant operations for futex +%define FUTEX_WAIT 0 %define FUTEX_PRIVATE_FLAG 0x80 @@ -38,8 +38,8 @@ section .text ; Wait for thread to exit, save its return value, and clean up. Arguments: -; rdi: u32* = handle of the thread to wait for -; rsi: void** = where to put the void* returned by the thread +; rdi: struct{u32,u32}* = handle of the thread to wait for +; rsi: void** = where to put void* returned by thread ; Returns zero on success, or a standard error code. global linen_thread_finish linen_thread_finish: @@ -51,15 +51,15 @@ linen_thread_finish: mov eax, -22 ; (EINVAL = -22) test rdi, rdi - jz join_end ; rdi is NULL + jz finish_return ; rdi is NULL - ; rdi is nonzero, so let's just assume it's a valid pointer. - ; If that assumption is wrong we'll get a segmentation fault. + ; rdi is nonzero, so let's just assume it's a valid pointer; + ; if that assumption is wrong we'll get a segmentation fault. ; But we don't yet trust that [rdi] is a valid thread handle! - ; To verify this, we check the canary value stored at [rdi + 4]. + ; To verify this we check the canary value stored at [rdi + 4]. mov ecx, [rdi + 4] cmp ecx, 0xDEADBEEF - jnz join_end + jnz finish_return ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ;;;; Wait until thread is finished ;;;; @@ -68,15 +68,16 @@ linen_thread_finish: ; We'll clobber rsi if we need to set up a futex call mov r8, rsi + finish_retry: ; When spawning, we set CLONE_CHILD_SETTID and CLONE_CHILD_CLEARTID: ; [rdi] contains the child thread's TID, and will get automatically ; cleared (to 0) when the child exits; this is what we'll watch for. ; Atomically check whether the target thread is still running. - ; if ([rdi] == 0) { goto join_done; } else { eax = [rdi]; } + ; if ([rdi] == 0) { goto finish_success; } else { eax = [rdi]; } xor eax, eax lock cmpxchg [rdi], eax - jz join_done + jz finish_success ; The thread is still busy, so block until it's done. ; The futex system call waits until the dword at an @@ -84,7 +85,7 @@ linen_thread_finish: ; See: man 2 futex ; futex: rdi = uaddr: address of the dword to watch - ; futex: rsi = futex_op: which futex operation we want + ; futex: rsi = futex_op: which futex operation we want: ; - FUTEX_WAIT: block until the value at [rdi] changes ; - FUTEX_PRIVATE_FLAG: FIXME waits forever, I don't understand why mov esi, FUTEX_WAIT @@ -93,21 +94,26 @@ linen_thread_finish: ; futex: r10 = timeout: in case we had a deadline (we don't) xor r10, r10 ; futex: r8 = uaddr2: ignored when FUTEX_WAIT is used - ; futex: r9 = val3: ignored when FUTEX WAIT is used + ; futex: r9 = val3: ignored when FUTEX_WAIT is used ; futex: rax = system call ID mov eax, SYS_FUTEX ; futex: rax = futex(rdi, rsi, rdx, r10, (r8), (r9)) syscall - ; Check result of futex: negative means failure + ; Sometimes the thread exits after the "lock cmpxchg" instruction + ; but before the futex call. In that case, futex returns EAGAIN. + cmp rax, -11 ; (EAGAIN = -11) + je finish_retry + + ; Any other nonzero return value means failure test rax, rax - jnz join_end + jnz finish_return ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ;;;; Clean up after thread's exit ;;;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; - join_done: + finish_success: ; The thread left its function return value on the stack, read it mov rdx, [rdi - 8] @@ -127,15 +133,15 @@ linen_thread_finish: ; Check result of munmap: nonzero means failure test rax, rax - jnz join_end + jnz finish_return ; Check if caller gave a location (r8) to save the return value (rdx) test r8, r8 - jz join_end ; caller doesn't care: gave NULL pointer + jz finish_return ; caller doesn't care: gave NULL pointer mov [r8], rdx ; Note: if munmap failed, the buffer is still there, so we ; can safely return an error without losing the return value. - join_end: + finish_return: ret |