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pipe: thundering herd problem in pipelock

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All reads and writes are serialized with a hand-rolled lock, but unlocking it
always wakes up all waiters. Existing flag fields get resized to make room for
introduction of waiter counter without growing the struct.

Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D27273
This commit is contained in:
Mateusz Guzik 2020-11-19 19:25:47 +00:00
parent 7e9e52e7a7
commit f9fe7b28bc
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=367852
2 changed files with 13 additions and 6 deletions
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14
sys/kern/sys_pipe.c
View File

@ -622,9 +622,13 @@ pipelock(struct pipe *cpipe, int catch)
if (catch)
prio |= PCATCH;
while (cpipe->pipe_state & PIPE_LOCKFL) {
cpipe->pipe_state |= PIPE_LWANT;
KASSERT(cpipe->pipe_waiters >= 0,
("%s: bad waiter count %d", __func__,
cpipe->pipe_waiters));
cpipe->pipe_waiters++;
error = msleep(cpipe, PIPE_MTX(cpipe),
prio, "pipelk", 0);
cpipe->pipe_waiters--;
if (error != 0)
return (error);
}
@ -642,10 +646,12 @@ pipeunlock(struct pipe *cpipe)
PIPE_LOCK_ASSERT(cpipe, MA_OWNED);
KASSERT(cpipe->pipe_state & PIPE_LOCKFL,
("Unlocked pipe passed to pipeunlock"));
KASSERT(cpipe->pipe_waiters >= 0,
("%s: bad waiter count %d", __func__,
cpipe->pipe_waiters));
cpipe->pipe_state &= ~PIPE_LOCKFL;
if (cpipe->pipe_state & PIPE_LWANT) {
cpipe->pipe_state &= ~PIPE_LWANT;
wakeup(cpipe);
if (cpipe->pipe_waiters > 0) {
wakeup_one(cpipe);
}
}

5
sys/sys/pipe.h
View File

@ -116,9 +116,10 @@ struct pipe {
struct pipe *pipe_peer; /* link with other direction */
struct pipepair *pipe_pair; /* container structure pointer */
u_short pipe_state; /* pipe status info */
u_short pipe_type; /* pipe type info */
u_char pipe_type; /* pipe type info */
u_char pipe_present; /* still present? */
int pipe_waiters; /* pipelock waiters */
int pipe_busy; /* busy flag, mostly to handle rundown sanely */
int pipe_present; /* still present? */
int pipe_wgen; /* writer generation for named pipe */
ino_t pipe_ino; /* fake inode for stat(2) */
};