Re: [PATCH AUTOSEL 5.5 542/542] pipe: use exclusive waits when reading or writing

[Linus Torvalds]

On Tue, Feb 18, 2020 at 9:54 AM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> Anyway, new patch attached. This hasn't been tested either, but I'll
> let you know if it's broken too ;)

That one looks good. Some small cosmetic edits later, and with a
commit log it looks like the appended.

If you're testing this and it works for your full CRIU test case too,
I'll add your tested-by if I get it before I end up pushing things out
later today,

                    Linus
From 7e2fe96d1b760eb3e7dec771db7b8983d5b6c25c Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Date: Tue, 18 Feb 2020 10:12:58 -0800
Subject: [PATCH] pipe: make sure to wake up everybody when the last
 reader/writer closes

Andrei Vagin reported that commit 0ddad21d3e99 ("pipe: use exclusive
waits when reading or writing") broke one of the CRIU tests.  He even
has a trivial reproducer:

    #include <unistd.h>
    #include <sys/types.h>
    #include <sys/wait.h>

    int main()
    {
            int p[2];
            pid_t p1, p2;
            int status;

            if (pipe(p) == -1)
                    return 1;

            p1 = fork();
            if (p1 == 0) {
                    close(p[1]);
                    read(p[0], &status, sizeof(status));
                    return 0;
            }
            p2 = fork();
            if (p2 == 0) {
                    close(p[1]);
                    read(p[0], &status, sizeof(status));
                    return 0;
            }
            sleep(1);
            close(p[1]);
            wait(&status);
            wait(&status);

            return 0;
    }

and the problem - once he points it out - is obvious.  We use these nice
exclusive waits, but when the last writer goes away, it then needs to
wake up _every_ reader (and conversely, the last reader disappearing
needs to wake every writer, of course).

In fact, when going through this, we had several small oddities around
how to wake things.  We did in fact wake every reader when we changed
the size of the pipe buffers.  But that's entirely pointless, since that
just acts as a possible source of new space - no new data to read.

And when we change the size of the buffer, we don't need to wake all
writers even when we add space - that case acts just as if somebody made
space by reading, and any writer that finds itself not filling it up
entirely will wake the next one.

On the other hand, on the exit path, we tried to limit the wakeups with
the proper poll keys etc, which is entirely pointless, because at that
point we obviously need to wake up everybody.  So don't do that: just
wake up everybody - but only do that if the counts changed to zero.

So fix those non-IO wakeups to be more proper: space change doesn't add
any new data, but it might make room for writers, so it wakes up a
writer.  And the actual changes to reader/writer counts should wake up
everybody, since everybody is affected (ie readers will all see EOF if
the writers have gone away, and writers will all get EPIPE if all
readers have gone away).

Fixes: 0ddad21d3e99 ("pipe: use exclusive waits when reading or writing")
Reported-by: Andrei Vagin <avagin@xxxxxxxxx>
Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
---
 fs/pipe.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 5a34d6c22d4c..2144507447c5 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -722,9 +722,10 @@ pipe_release(struct inode *inode, struct file *file)
 	if (file->f_mode & FMODE_WRITE)
 		pipe->writers--;
 
-	if (pipe->readers || pipe->writers) {
-		wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM | EPOLLERR | EPOLLHUP);
-		wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM | EPOLLERR | EPOLLHUP);
+	/* Was that the last reader or writer, but not the other side? */
+	if (!pipe->readers != !pipe->writers) {
+		wake_up_interruptible_all(&pipe->rd_wait);
+		wake_up_interruptible_all(&pipe->wr_wait);
 		kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
 		kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
 	}
@@ -1026,8 +1027,8 @@ static int wait_for_partner(struct pipe_inode_info *pipe, unsigned int *cnt)
 
 static void wake_up_partner(struct pipe_inode_info *pipe)
 {
-	wake_up_interruptible(&pipe->rd_wait);
-	wake_up_interruptible(&pipe->wr_wait);
+	wake_up_interruptible_all(&pipe->rd_wait);
+	wake_up_interruptible_all(&pipe->wr_wait);
 }
 
 static int fifo_open(struct inode *inode, struct file *filp)
@@ -1144,7 +1145,7 @@ static int fifo_open(struct inode *inode, struct file *filp)
 
 err_wr:
 	if (!--pipe->writers)
-		wake_up_interruptible(&pipe->rd_wait);
+		wake_up_interruptible_all(&pipe->rd_wait);
 	ret = -ERESTARTSYS;
 	goto err;
 
@@ -1271,8 +1272,9 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
 	pipe->max_usage = nr_slots;
 	pipe->tail = tail;
 	pipe->head = head;
-	wake_up_interruptible_all(&pipe->rd_wait);
-	wake_up_interruptible_all(&pipe->wr_wait);
+
+	/* This might have made more room for writers */
+	wake_up_interruptible(&pipe->wr_wait);
 	return pipe->max_usage * PAGE_SIZE;
 
 out_revert_acct:
-- 
2.24.0.158.g3fed155289