runtime: drop SIGPROF while in ARM < 7 kernel helpers

On Linux ARMv6 and below runtime/internal/atomic.Cas calls into a kernel
cas helper at a fixed address. If a SIGPROF arrives while executing the
kernel helper, the sigprof lostAtomic logic will miss that we are
potentially in the spinlock critical section, which could cause
a deadlock when using atomics later in sigprof.

Fixes golang#47505

Change-Id: If8ba0d0fc47e45d4e6c68eca98fac4c6ed4e43c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/341889
Trust: Michael Pratt <[email protected]>
Run-TryBot: Michael Pratt <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
TryBot-Result: Go Bot <[email protected]>

src/runtime/proc.go

@@ -4619,7 +4619,7 @@ func sigprof(pc, sp, lr uintptr, gp *g, mp *m) {
  return
  }
 
- // On mips{,le}, 64bit atomics are emulated with spinlocks, in
+ // On mips{,le}/arm, 64bit atomics are emulated with spinlocks, in
  // runtime/internal/atomic. If SIGPROF arrives while the program is inside
  // the critical section, it creates a deadlock (when writing the sample).
  // As a workaround, create a counter of SIGPROFs while in critical section
@@ -4632,6 +4632,13 @@ func sigprof(pc, sp, lr uintptr, gp *g, mp *m) {
  return
  }
  }
+ if GOARCH == "arm" && goarm < 7 && GOOS == "linux" && pc&0xffff0000 == 0xffff0000 {
+ // runtime/internal/atomic functions call into kernel
+ // helpers on arm < 7. See
+ // runtime/internal/atomic/sys_linux_arm.s.
+ cpuprof.lostAtomic++
+ return
+ }
  }
 
  // Profiling runs concurrently with GC, so it must not allocate.