linux-kernel - Re: [PATCH -mm 3/5] fault-inject: make fail-nth read/write interface symmetric

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Date:   Wed, 12 Jul 2017 13:49:38 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Akinobu Mita <akinobu.mita@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm 3/5] fault-inject: make fail-nth read/write
 interface symmetric

On Fri, 7 Apr 2017 22:37:01 +0200 Dmitry Vyukov <dvyukov@...gle.com> wrote:

> On Thu, Apr 6, 2017 at 4:55 PM, Akinobu Mita <akinobu.mita@...il.com> wrote:
> > The read interface for fail-nth looks a bit odd.  Read from this file
> > returns "NYYYY..." or "YYYYY..." (this makes me surprise when cat this
> > file).  Because there is no EOF condition. The first character indicates
> > current->fail_nth is zero or not, and then current->fail_nth is reset
> > to zero.
> >
> > Just returning task->fail_nth value is more natural to understand.
> >
> > Cc: Dmitry Vyukov <dvyukov@...gle.com>
> > Signed-off-by: Akinobu Mita <akinobu.mita@...il.com>
> > ---
> >  Documentation/fault-injection/fault-injection.txt | 13 +++++++------
> >  fs/proc/base.c                                    | 14 ++++++--------
> >  2 files changed, 13 insertions(+), 14 deletions(-)
> >
> > diff --git a/Documentation/fault-injection/fault-injection.txt b/Documentation/fault-injection/fault-injection.txt
> > index a321905..370ddcb 100644
> > --- a/Documentation/fault-injection/fault-injection.txt
> > +++ b/Documentation/fault-injection/fault-injection.txt
> > @@ -139,9 +139,9 @@ o proc entries
> >  - /proc/self/task/<current-tid>/fail-nth:
> >
> >         Write to this file of integer N makes N-th call in the task fail.
> > -       Read from this file returns a single char 'Y' or 'N'
> > -       that says if the fault setup with a previous write to this file was
> > -       injected or not, and disables the fault if it wasn't yet injected.
> > +       Read from this file returns a integer value. A value of '0' indicates
> > +       that the fault setup with a previous write to this file was injected.
> > +       A positive integer N indicates that the fault wasn't yet injected.
> >         Note that this file enables all types of faults (slab, futex, etc).
> >         This setting takes precedence over all other generic debugfs settings
> >         like probability, interval, times, etc. But per-capability settings
> > @@ -325,13 +325,14 @@ int main()
> >                 write(fail_nth, buf, strlen(buf));
> >                 res = socketpair(AF_LOCAL, SOCK_STREAM, 0, fds);
> >                 err = errno;
> > -               read(fail_nth, buf, 1);
> > +               pread(fail_nth, buf, sizeof(buf), 0);
> >                 if (res == 0) {
> >                         close(fds[0]);
> >                         close(fds[1]);
> >                 }
> > -               printf("%d-th fault %c: res=%d/%d\n", i, buf[0], res, err);
> > -               if (buf[0] != 'Y')
> > +               printf("%d-th fault %c: res=%d/%d\n", i, atoi(buf) ? 'N' : 'Y',
> > +                       res, err);
> > +               if (atoi(buf))
> >                         break;
> >         }
> >         return 0;
> > diff --git a/fs/proc/base.c b/fs/proc/base.c
> > index 42c52e2..9d14215 100644
> > --- a/fs/proc/base.c
> > +++ b/fs/proc/base.c
> > @@ -1383,7 +1383,8 @@ static ssize_t proc_fail_nth_read(struct file *file, char __user *buf,
> >                                   size_t count, loff_t *ppos)
> >  {
> >         struct task_struct *task;
> > -       int err;
> > +       char numbuf[PROC_NUMBUF];
> > +       ssize_t len;
> >
> >         task = get_proc_task(file_inode(file));
> >         if (!task)
> > @@ -1391,13 +1392,10 @@ static ssize_t proc_fail_nth_read(struct file *file, char __user *buf,
> >         put_task_struct(task);
> >         if (task != current)
> >                 return -EPERM;
> > -       if (count < 1)
> > -               return -EINVAL;
> > -       err = put_user((char)(current->fail_nth ? 'N' : 'Y'), buf);
> > -       if (err)
> > -               return err;
> > -       current->fail_nth = 0;
> > -       return 1;
> > +       len = snprintf(numbuf, sizeof(numbuf), "%u\n", task->fail_nth);
> 
> If we allow setting this for non current task, then we need to prevent
> data races as the task uses task->fail_nth concurrently. Reads then
> should use READ_ONCE and writes in fault-inject.c should use
> WRITE_ONCE.

This remains unresolved?