runtime: concurrent map reads grow map and corrupt map internals

[bradfitz]

The old hashmap allowed concurrent reads.

The new hashmap code didn't, and led to crashes when GOMAXPROCS > 1.

From khr:

-----------
So I think we've found the problem.  It is triggered by two parallel reads to a map.  My
hashmap implementation was assuming that wouldn't happen.  Internally, the hash table is
grown lazily and I was doing a little bit of growing work during each read.  That
growing work can't happen in parallel and causes corruption.

The quick fix is to disable growing work on reads.  I'll try to get that in today.  The
longer fix would be to have some internal synchronization for the growing work.
------------

Several other bugs may or may not be this issue.

[bradfitz]

Comment 1:

This issue was updated by revision d76f28f.

R=golang-dev, khr
CC=golang-dev
https://golang.org/cl/8222044

[randall77]

Comment 2:

Should be fixed by revision https://code.google.com/p/go/source/detail?r=af469280a34b

Status changed to Fixed.