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runtime: fix crash during cpu profiling
mp->mcache can be concurrently modified by runtime·helpgc. In such case sigprof can remember mcache=nil, then helpgc sets it to non-nil, then sigprof restores it back to nil, GC crashes with nil mcache. R=rsc CC=golang-codereviews https://golang.org/cl/58860044
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@ -2115,7 +2115,6 @@ runtime·sigprof(uint8 *pc, uint8 *sp, uint8 *lr, G *gp, M *mp)
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{
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int32 n;
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bool traceback;
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MCache *mcache;
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// Do not use global m in this function, use mp instead.
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// On windows one m is sending reports about all the g's, so m means a wrong thing.
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byte m;
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@ -2127,8 +2126,7 @@ runtime·sigprof(uint8 *pc, uint8 *sp, uint8 *lr, G *gp, M *mp)
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return;
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// Profiling runs concurrently with GC, so it must not allocate.
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mcache = mp->mcache;
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mp->mcache = nil;
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mp->mallocing++;
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// Define that a "user g" is a user-created goroutine, and a "system g"
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// is one that is m->g0 or m->gsignal. We've only made sure that we
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@ -2216,7 +2214,7 @@ runtime·sigprof(uint8 *pc, uint8 *sp, uint8 *lr, G *gp, M *mp)
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runtime·lock(&prof);
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if(prof.fn == nil) {
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runtime·unlock(&prof);
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mp->mcache = mcache;
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mp->mallocing--;
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return;
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}
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n = 0;
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@ -2229,7 +2227,7 @@ runtime·sigprof(uint8 *pc, uint8 *sp, uint8 *lr, G *gp, M *mp)
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}
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prof.fn(prof.pcbuf, n);
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runtime·unlock(&prof);
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mp->mcache = mcache;
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mp->mallocing--;
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}
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// Arrange to call fn with a traceback hz times a second.
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