runtime: add helper for getting an mcache in allocation contexts

This change adds a function getMCache which returns the current P's
mcache if it's available, and otherwise tries to get mcache0 if we're
bootstrapping. This function will come in handy as we need to replicate
this behavior in multiple places in future changes.

Change-Id: I536073d6f6dc6c6390269e613ead9f8bcb6e7f98
Reviewed-on: https://go-review.googlesource.com/c/go/+/246976
Run-TryBot: Michael Knyszek <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Trust: Michael Knyszek <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>

Author: mknyszek

src/runtime/malloc.go

@@ -972,19 +972,7 @@ func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
 
 	shouldhelpgc := false
 	dataSize := size
-	var c *mcache
-	if mp.p != 0 {
-		c = mp.p.ptr().mcache
-	} else {
-		// We will be called without a P while bootstrapping,
-		// in which case we use mcache0, which is set in mallocinit.
-		// mcache0 is cleared when bootstrapping is complete,
-		// by procresize.
-		c = mcache0
-		if c == nil {
-			throw("malloc called with no P")
-		}
-	}
+	c := getMCache()
 	var span *mspan
 	var x unsafe.Pointer
 	noscan := typ == nil || typ.ptrdata == 0
@@ -1212,16 +1200,7 @@ func reflect_unsafe_NewArray(typ *_type, n int) unsafe.Pointer {
 }
 
 func profilealloc(mp *m, x unsafe.Pointer, size uintptr) {
-	var c *mcache
-	if mp.p != 0 {
-		c = mp.p.ptr().mcache
-	} else {
-		c = mcache0
-		if c == nil {
-			throw("profilealloc called with no P")
-		}
-	}
-	c.nextSample = nextSample()
+	getMCache().nextSample = nextSample()
 	mProf_Malloc(x, size)
 }
 

src/runtime/mcache.go

@@ -131,6 +131,29 @@ func freemcache(c *mcache, recipient *mcache) {
 	})
 }
 
+// getMCache is a convenience function which tries to obtain an mcache.
+//
+// Must be running with a P when called (so the caller must be in a
+// non-preemptible state) or must be called during bootstrapping.
+func getMCache() *mcache {
+	// Grab the mcache, since that's where stats live.
+	pp := getg().m.p.ptr()
+	var c *mcache
+	if pp == nil {
+		// We will be called without a P while bootstrapping,
+		// in which case we use mcache0, which is set in mallocinit.
+		// mcache0 is cleared when bootstrapping is complete,
+		// by procresize.
+		c = mcache0
+		if c == nil {
+			throw("getMCache called with no P or outside bootstrapping")
+		}
+	} else {
+		c = pp.mcache
+	}
+	return c
+}
+
 // donate flushes data and resources which have no global
 // pool to another mcache.
 func (c *mcache) donate(d *mcache) {