runtime/race: false negative on fmt.Fprintf

[dvyukov]

The race in the following program is detected:
http://play.golang.org/p/tq5NFOk5_5

However a similar race is not detected in:
http://play.golang.org/p/veGvrmUKn7

[dvyukov]

Comment 2:

The problem is in the fmt package internal synchronized cache. When the goroutines are
executes sequentially one-by-one, the cache's mutex looks exactly as if it synchronizes
accesses to the buf.
If I increase number of goroutines to 1000 and runt with GOMAXPROCS=16, then the race is
detected -- some goroutines happen to run concurrently in the racy section of code.
sync.Pool would help here, because it may expose less synchronization to the race
detector.
Another counter-measure can be to introduce more random preemption when running under
race detector.

[rsc]

Comment 3:

Labels changed: added go1.3maybe.

[rsc]

Comment 4:

Labels changed: added release-none, removed go1.3maybe.

[rsc]

Comment 5:

Labels changed: added repo-main.

[dvyukov]

Comment 6:

This is fixed by sync.Pool changes. Now the race is detected reliably.

Status changed to Fixed.