A race exists between the parent and child processes after a fork.
The child needs to access the new M pointer passed as an argument
but the parent may have already returned and clobbered it.
Previously, we avoided this by saving the necessary data into
registers before the rfork system call but this isn't guaranteed
to work because Plan 9 makes no promises about the register state
after a system call. Only the 386 kernel seems to save them.
For amd64 and arm, this method won't work.
We eliminate the race by allocating stack space for the scheduler
goroutines (g0) in the per-process copy-on-write stack segment and
by only calling rfork on the scheduler stack.
LGTM=aram, 0intro, rsc
R=aram, 0intro, mischief, rsc
CC=golang-codereviews
https://golang.org/cl/
110680044
void
runtime·newosproc(M *mp, void *stk)
{
- mp->tls[0] = mp->id; // so 386 asm can find it
- if(0){
- runtime·printf("newosproc stk=%p m=%p g=%p rfork=%p id=%d/%d ostk=%p\n",
- stk, mp, mp->g0, runtime·rfork, mp->id, (int32)mp->tls[0], &mp);
- }
+ int32 pid;
+
+ if(0)
+ runtime·printf("newosproc mp=%p ostk=%p\n", mp, &mp);
- if(runtime·rfork(RFPROC|RFMEM|RFNOWAIT, stk, mp, mp->g0, runtime·mstart) < 0)
- runtime·throw("newosproc: rfork failed");
+ USED(stk);
+ if((pid = runtime·rfork(RFPROC|RFMEM|RFNOWAIT)) < 0)
+ runtime·throw("newosproc: rfork failed\n");
+ if(pid == 0)
+ runtime·tstart_plan9(mp);
}
#pragma textflag NOSPLIT
func exits(msg *byte)
func brk_(addr unsafe.Pointer) uintptr
func sleep(ms int32) int32
-func rfork(flags int32, stk, mm, gg, fn unsafe.Pointer) int32
+func rfork(flags int32) int32
func plan9_semacquire(addr *uint32, block int32) int32
func plan9_tsemacquire(addr *uint32, ms int32) int32
func plan9_semrelease(addr *uint32, count int32) int32
func nsec(*int64) int64
func sigtramp(ureg, msg unsafe.Pointer)
func setfpmasks()
+func tstart_plan9(newm *m)
func errstr() string
// The size of the note handler frame varies among architectures,
void runtime·exits(int8* msg);
intptr runtime·brk_(void*);
int32 runtime·sleep(int32 ms);
-int32 runtime·rfork(int32 flags, void *stk, M *mp, G *gp, void (*fn)(void));
+int32 runtime·rfork(int32 flags);
int32 runtime·plan9_semacquire(uint32 *addr, int32 block);
int32 runtime·plan9_tsemacquire(uint32 *addr, int32 ms);
int32 runtime·plan9_semrelease(uint32 *addr, int32 count);
void runtime·sigpanic(void);
void runtime·goexitsall(int8*);
void runtime·setfpmasks(void);
+void runtime·tstart_plan9(M *newm);
/* open */
enum
mcommoninit(mp);
// In case of cgo or Solaris, pthread_create will make us a stack.
- // Windows will layout sched stack on OS stack.
- if(runtime·iscgo || Solaris || Windows)
+ // Windows and Plan 9 will layout sched stack on OS stack.
+ if(runtime·iscgo || Solaris || Windows || Plan9)
mp->g0 = runtime·malg(-1);
else
mp->g0 = runtime·malg(8192);
Solaris = 0
};
#endif
+#ifdef GOOS_plan9
+enum {
+ Plan9 = 1
+};
+#else
+enum {
+ Plan9 = 0
+};
+#endif
// Lock-free stack node.
struct LFNode
INT $64
MOVL AX, ret+8(FP)
RET
-
-TEXT runtime·rfork(SB),NOSPLIT,$0
- MOVL $19, AX // rfork
- MOVL stack+8(SP), CX
- MOVL mm+12(SP), BX // m
- MOVL gg+16(SP), DX // g
- MOVL fn+20(SP), SI // fn
- INT $64
- // In parent, return.
- CMPL AX, $0
- JEQ 3(PC)
- MOVL AX, ret+20(FP)
+TEXT runtime·rfork(SB),NOSPLIT,$0
+ MOVL $19, AX
+ INT $64
+ MOVL AX, ret+4(FP)
RET
- // set SP to be on the new child stack
- MOVL CX, SP
+TEXT runtime·tstart_plan9(SB),NOSPLIT,$0
+ MOVL newm+0(FP), CX
+ MOVL m_g0(CX), DX
- // Initialize m, g.
- get_tls(AX)
- MOVL DX, g(AX)
- MOVL BX, g_m(DX)
+ // Layout new m scheduler stack on os stack.
+ MOVL SP, AX
+ MOVL AX, (g_stack+stack_hi)(DX)
+ SUBL $(64*1024), AX // stack size
+ MOVL AX, (g_stack+stack_lo)(DX)
+ MOVL AX, g_stackguard0(DX)
+ MOVL AX, g_stackguard1(DX)
// Initialize procid from TOS struct.
MOVL _tos(SB), AX
- MOVL 48(AX), AX // procid
- MOVL AX, m_procid(BX) // save pid as m->procid
-
+ MOVL 48(AX), AX
+ MOVL AX, m_procid(CX) // save pid as m->procid
+
+ // Finally, initialize g.
+ get_tls(BX)
+ MOVL DX, g(BX)
+
CALL runtime·stackcheck(SB) // smashes AX, CX
-
- MOVL 0(DX), DX // paranoia; check they are not nil
- MOVL 0(BX), BX
-
- // more paranoia; check that stack splitting code works
- PUSHL SI
- CALL runtime·emptyfunc(SB)
- POPL SI
-
- CALL SI // fn()
- CALL runtime·exit(SB)
- MOVL AX, ret+20(FP)
+ CALL runtime·mstart(SB)
+
+ MOVL $0x1234, 0x1234 // not reached
RET
// void sigtramp(void *ureg, int8 *note)
RET
TEXT runtime·rfork(SB),NOSPLIT,$0
- MOVQ $19, BP // rfork
+ MOVQ $19, BP
SYSCALL
-
- // In parent, return.
- CMPQ AX, $0
- JEQ 3(PC)
- MOVL AX, ret+40(FP)
+ MOVL AX, ret+8(FP)
RET
- // In child on forked stack.
- MOVQ mm+24(SP), BX // m
- MOVQ gg+32(SP), DX // g
- MOVQ fn+40(SP), SI // fn
-
- // set SP to be on the new child stack
- MOVQ stack+16(SP), CX
- MOVQ CX, SP
+TEXT runtime·tstart_plan9(SB),NOSPLIT,$0
+ MOVQ newm+0(FP), CX
+ MOVQ m_g0(CX), DX
- // Initialize m, g.
- get_tls(AX)
- MOVQ DX, g(AX)
- MOVQ BX, g_m(DX)
+ // Layout new m scheduler stack on os stack.
+ MOVQ SP, AX
+ MOVQ AX, (g_stack+stack_hi)(DX)
+ SUBQ $(64*1024), AX // stack size
+ MOVQ AX, (g_stack+stack_lo)(DX)
+ MOVQ AX, g_stackguard0(DX)
+ MOVQ AX, g_stackguard1(DX)
// Initialize procid from TOS struct.
MOVQ _tos(SB), AX
MOVQ 64(AX), AX
- MOVQ AX, m_procid(BX) // save pid as m->procid
-
+ MOVQ AX, m_procid(CX) // save pid as m->procid
+
+ // Finally, initialize g.
+ get_tls(BX)
+ MOVQ DX, g(BX)
+
CALL runtime·stackcheck(SB) // smashes AX, CX
-
- MOVQ 0(DX), DX // paranoia; check they are not nil
- MOVQ 0(BX), BX
-
- CALL SI // fn()
- CALL runtime·exit(SB)
- MOVL AX, ret+40(FP)
+ CALL runtime·mstart(SB)
+
+ MOVQ $0x1234, 0x1234 // not reached
RET
// This is needed by asm_amd64.s