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.
Modified files:
Index: src/sys/kern/sys_pipe.c
diff -u src/sys/kern/sys_pipe.c:1.151 src/sys/kern/sys_pipe.c:1.152
--- src/sys/kern/sys_pipe.c:1.151 Fri Dec 11 03:00:09 2020
+++ src/sys/kern/sys_pipe.c Mon Jan 25 19:21:11 2021
@@ -1,4 +1,4 @@
-/* $NetBSD: sys_pipe.c,v 1.151 2020/12/11 03:00:09 thorpej Exp $ */
+/* $NetBSD: sys_pipe.c,v 1.152 2021/01/25 19:21:11 dholland Exp $ */
/*-
* Copyright (c) 2003, 2007, 2008, 2009 The NetBSD Foundation, Inc.
@@ -68,7 +68,7 @@
*/
#include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: sys_pipe.c,v 1.151 2020/12/11 03:00:09 thorpej Exp $");
+__KERNEL_RCSID(0, "$NetBSD: sys_pipe.c,v 1.152 2021/01/25 19:21:11 dholland Exp $");
#include <sys/param.h>
#include <sys/systm.h>
@@ -344,13 +344,19 @@ pipelock(struct pipe *pipe, bool catch_p
KASSERT(mutex_owned(pipe->pipe_lock));
while (pipe->pipe_state & PIPE_LOCKFL) {
- pipe->pipe_state |= PIPE_LWANT;
+ pipe->pipe_waiters++;
+ KASSERT(pipe->pipe_waiters != 0); /* just in case */
if (catch_p) {
error = cv_wait_sig(&pipe->pipe_lkcv, pipe->pipe_lock);
- if (error != 0)
+ if (error != 0) {
+ KASSERT(pipe->pipe_waiters > 0);
+ pipe->pipe_waiters--;
return error;
+ }
} else
cv_wait(&pipe->pipe_lkcv, pipe->pipe_lock);
+ KASSERT(pipe->pipe_waiters > 0);
+ pipe->pipe_waiters--;
}
pipe->pipe_state |= PIPE_LOCKFL;
@@ -368,9 +374,8 @@ pipeunlock(struct pipe *pipe)
KASSERT(pipe->pipe_state & PIPE_LOCKFL);
pipe->pipe_state &= ~PIPE_LOCKFL;
- if (pipe->pipe_state & PIPE_LWANT) {
- pipe->pipe_state &= ~PIPE_LWANT;
- cv_broadcast(&pipe->pipe_lkcv);
+ if (pipe->pipe_waiters > 0) {
+ cv_signal(&pipe->pipe_lkcv);
}
}
Index: src/sys/sys/pipe.h
diff -u src/sys/sys/pipe.h:1.37 src/sys/sys/pipe.h:1.38
--- src/sys/sys/pipe.h:1.37 Thu Jun 25 14:22:19 2020
+++ src/sys/sys/pipe.h Mon Jan 25 19:21:11 2021
@@ -1,4 +1,4 @@
-/* $NetBSD: pipe.h,v 1.37 2020/06/25 14:22:19 jdolecek Exp $ */
+/* $NetBSD: pipe.h,v 1.38 2021/01/25 19:21:11 dholland Exp $ */
/*
* Copyright (c) 1996 John S. Dyson
@@ -80,8 +80,7 @@ struct pipebuf {
#define PIPE_SIGNALR 0x020 /* Do selwakeup() on read(2) */
#define PIPE_LOCKFL 0x100 /* Process has exclusive access to
pointers/data. */
-#define PIPE_LWANT 0x200 /* Process wants exclusive access to
- pointers/data. */
+/* unused 0x200 */
#define PIPE_RESTART 0x400 /* Return ERESTART to blocked syscalls */
/*
@@ -100,6 +99,7 @@ struct pipe {
struct timespec pipe_mtime; /* time of last modify */
struct timespec pipe_btime; /* time of creation */
pid_t pipe_pgid; /* process group for sigio */
+ u_int pipe_waiters; /* number of waiters pending */
struct pipe *pipe_peer; /* link with other direction */
u_int pipe_state; /* pipe status info */
int pipe_busy; /* busy flag, to handle rundown */
