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

[Andrei Vagin <avagin@gmail.com>]

Hello Linus,

After this change, one more criu test became flaky. This is due to one
of corner cases, so I am not sure that we need to fix something in the
kernel. I have fixed this issue in the test. I am not sure that this
will affect any real applications.

Here is the reproducer:

#include <unistd.h>
#include <stdio.h>

int main()
{
    char buf[1<<20];
    int pid, p[2], ret;

    if (pipe(p) < 0)
        return 1;
    pid = fork();
    if (pid == 0) {
        close(p[1]);

        ret = read(p[0], buf, sizeof(buf));
        if (ret < 0)
            return 1;
        printf("read -> %d\n", ret);
        return 0;
    }
    close(p[0]);
    ret = write(p[1], buf, sizeof(buf));
    if (ret < 0)
        return 1;
    printf("write -> %d\n", ret);
    return 0;
}

Before this change:
[avagin@laptop fifo]$ uname -a
Linux laptop 5.3.7-200.fc30.x86_64 #1 SMP Fri Oct 18 20:13:59 UTC 2019
x86_64 x86_64 x86_64 GNU/Linux
[avagin@laptop fifo]$ strace -e read,write,pipe -f ./pipe_bigbuf
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\260r\2\0\0\0\0\0"...,
832) = 832
read(3, "\6\0\0\0\4\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0"...,
784) = 784
read(3, "\4\0\0\0\20\0\0\0\5\0\0\0GNU\0\2\0\0\300\4\0\0\0\3\0\0\0\0\0\0\0",
32) = 32
read(3, "\4\0\0\0\24\0\0\0\3\0\0\0GNU\0gZ\316<\240z\v\206=\360\37F\32{\t\204"...,
68) = 68
read(3, "\6\0\0\0\4\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0"...,
784) = 784
read(3, "\4\0\0\0\20\0\0\0\5\0\0\0GNU\0\2\0\0\300\4\0\0\0\3\0\0\0\0\0\0\0",
32) = 32
read(3, "\4\0\0\0\24\0\0\0\3\0\0\0GNU\0gZ\316<\240z\v\206=\360\37F\32{\t\204"...,
68) = 68
pipe([3, 4])                            = 0
strace: Process 622350 attached
[pid 622349] write(4,
"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
1048576 <unfinished ...>
[pid 622350] read(3,  <unfinished ...>
[pid 622349] <... write resumed> )      = 1048576
[pid 622350] <... read resumed>
"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
1048576) = 1048576
[pid 622349] write(1, "write -> 1048576\n", 17write -> 1048576
) = 17
[pid 622350] write(1, "read -> 1048576\n", 16read -> 1048576
) = 16
[pid 622349] +++ exited with 0 +++
+++ exited with 0 +++

After this change:
[root@fc24 ~]# strace -e read,write,pipe -f ./pipe_bigbuf
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\260r\2\0\0\0\0\0"...,
832) = 832
read(3, "\6\0\0\0\4\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0"...,
784) = 784
read(3, "\4\0\0\0\20\0\0\0\5\0\0\0GNU\0\2\0\0\300\4\0\0\0\3\0\0\0\0\0\0\0",
32) = 32
read(3, "\4\0\0\0\24\0\0\0\3\0\0\0GNU\0L\355\265_\4c\17r@ix\305q\26W\242"...,
68) = 68
read(3, "\6\0\0\0\4\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0"...,
784) = 784
read(3, "\4\0\0\0\20\0\0\0\5\0\0\0GNU\0\2\0\0\300\4\0\0\0\3\0\0\0\0\0\0\0",
32) = 32
read(3, "\4\0\0\0\24\0\0\0\3\0\0\0GNU\0L\355\265_\4c\17r@ix\305q\26W\242"...,
68) = 68
pipe([3, 4])                            = 0
strace: Process 4946 attached
[pid  4945] write(4,
"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
1048576 <unfinished ...>
[pid  4946] read(3,
"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
1048576) = 65536
[pid  4946] write(1, "read -> 65536\n", 14read -> 65536
) = 14
[pid  4945] <... write resumed>)        = 131072
[pid  4946] +++ exited with 0 +++
--- SIGPIPE {si_signo=SIGPIPE, si_code=SI_USER, si_pid=4945, si_uid=0} ---
+++ killed by SIGPIPE +++

On Tue, Feb 18, 2020 at 3:03 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Tue, Feb 18, 2020 at 2:33 PM Andrei Vagin <avagin@gmail.com> wrote:
> >
> > I run CRIU tests on the kernel with both these patches. Everything work
> > as expected.
>
> Thanks. I've added your tested-by and pushed out the fix.
>
>            Linus