CVS commit: src/sys

["David A. Holland" <dholland%netbsd.org@localhost>]

Module Name:    src
Committed By:   dholland
Date:           Mon Jan 25 19:21:11 UTC 2021

Modified Files:
        src/sys/kern: sys_pipe.c
        src/sys/sys: pipe.h

Log Message:
Fix a thundering herd problem in pipes.

Wake only one waiter when data becomes available, not all of them.
Waking them all is not a usual case, but turns up with make's job
token pipes. (Probably make's job signalling scheme should also be
revised, assuming rillig hasn't already done that, but that's a
separate issue.)

This change will not do us much good for the moment because we don't
distinguish cv_signal from cv_broadcast for interruptible sleeps, but
that's also a separate problem.

Seen on FreeBSD; from mjg at freebsd a couple months ago. Patch was
mine (iirc) but the real work in this sort of thing is discovering the
problem.


To generate a diff of this commit:
cvs rdiff -u -r1.151 -r1.152 src/sys/kern/sys_pipe.c
cvs rdiff -u -r1.37 -r1.38 src/sys/sys/pipe.h

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.