Re: [PATCH] block: allow for_each_bvec to support zero len bvec

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

On 2020/08/11 1:23, Ming Lei wrote:
> The same behavior can be observed on v4.8 too, both v4.8 and v4.18
> includes 1bdc76aea115. If you apply the fix against v4.8, you can
> observe the same behavior too.

(...snipped...)

> I think this new issue may be introduced between v4.18 and v5.8.

Bisection reported that both problems ("infinite busy loop lockup" and "premature splice() return") became
visible since commit a194dfe6e6f6f720 ("pipe: Rearrange sequence in pipe_write() to preallocate slot").

Therefore, although the bug might have been existed since commit 1bdc76aea115 ("iov_iter: use bvec iterator
to implement iterate_bvec()"), we need to apply your patch to 5.5+ only.

----- test case -----

#define _GNU_SOURCE
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>

int main(int argc, char *argv[])
{
        static char buffer[4096];
        const int fd = open("/tmp/testfile", O_WRONLY | O_CREAT, 0600);
        int pipe_fd[2] = { EOF, EOF };
        pipe(pipe_fd);
        write(pipe_fd[1], NULL, sizeof(buffer));
        write(pipe_fd[1], NULL, sizeof(buffer));
        memset(buffer, 'a', sizeof(buffer));
        if (argc > 1)
                write(pipe_fd[1], buffer, sizeof(buffer));
        write(pipe_fd[1], NULL, sizeof(buffer));
        write(pipe_fd[1], NULL, sizeof(buffer));
        memset(buffer, 'b', sizeof(buffer));
        if (argc > 1)
                write(pipe_fd[1], buffer, sizeof(buffer));
        write(pipe_fd[1], NULL, sizeof(buffer));
        write(pipe_fd[1], NULL, sizeof(buffer));
        memset(buffer, 'c', sizeof(buffer));
        if (argc > 1)
                write(pipe_fd[1], buffer, sizeof(buffer));
        write(pipe_fd[1], NULL, sizeof(buffer));
        write(pipe_fd[1], NULL, sizeof(buffer));
        memset(buffer, 'd', sizeof(buffer));
        if (argc > 1)
                write(pipe_fd[1], buffer, sizeof(buffer));
        write(pipe_fd[1], NULL, sizeof(buffer));
        write(pipe_fd[1], NULL, sizeof(buffer));
        splice(pipe_fd[0], NULL, fd, NULL, 65536, 0);
        return 0;
}

----- bisect log -----

# bad: [e42617b825f8073569da76dc4510bfa019b1c35a] Linux 5.5-rc1
# good: [219d54332a09e8d8741c1e1982f5eae56099de85] Linux 5.4
# good: [4d856f72c10ecb060868ed10ff1b1453943fc6c8] Linux 5.3
# good: [0ecfebd2b52404ae0c54a878c872bb93363ada36] Linux 5.2
# good: [e93c9c99a629c61837d5a7fc2120cd2b6c70dbdd] Linux 5.1
# good: [1c163f4c7b3f621efff9b28a47abb36f7378d783] Linux 5.0
git bisect start 'v5.5-rc1' 'v5.4' 'v5.3' 'v5.2' 'v5.1' 'v5.0' '--' 'fs/splice.c' 'fs/pipe.c' 'include/linux/splice.h' 'include/linux/pipe_fs_i.h'
# bad: [8f868d68d335a17923dffb6858f8e9b656424699] pipe: Fix missing mask update after pipe_wait()
git bisect bad 8f868d68d335a17923dffb6858f8e9b656424699
# bad: [a194dfe6e6f6f7205eea850a420f2bc6a1541209] pipe: Rearrange sequence in pipe_write() to preallocate slot
git bisect bad a194dfe6e6f6f7205eea850a420f2bc6a1541209
# good: [6718b6f855a0b4962d54bd625be2718cb820cec6] pipe: Allow pipes to have kernel-reserved slots
git bisect good 6718b6f855a0b4962d54bd625be2718cb820cec6
# good: [8446487feba988a92e7649c60367510f0b0445a8] pipe: Conditionalise wakeup in pipe_read()
git bisect good 8446487feba988a92e7649c60367510f0b0445a8
# first bad commit: [a194dfe6e6f6f7205eea850a420f2bc6a1541209] pipe: Rearrange sequence in pipe_write() to preallocate slot