linux-kernel - Re: [GIT PULL] pipe: Notification queue preparation

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Message-ID: <20191205172127.GW2734@suse.cz>
Date:   Thu, 5 Dec 2019 18:21:27 +0100
From:   David Sterba <dsterba@...e.cz>
To:     David Howells <dhowells@...hat.com>
Cc:     torvalds@...ux-foundation.org,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>, raven@...maw.net,
        Christian Brauner <christian@...uner.io>,
        keyrings@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-block@...r.kernel.org, linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [GIT PULL] pipe: Notification queue preparation

On Thu, Dec 05, 2019 at 01:56:57PM +0000, David Howells wrote:
> David Sterba <dsterba@...e.cz> wrote:
> 
> > [<0>] pipe_write+0x1be/0x4b0
> 
> Can you get me a line number of that?  Assuming you've built with -g, load
> vmlinux into gdb and do "i li pipe_write+0x1be".

I built it with -g (DEBUG_INFO) but there's no output for the command (gdb 8.2):

(gdb) i li pipe_write+0x1be
Function "pipe_write+0x1be" not defined.

But the address can tell something:

(gdb) l *(pipe_write+0x1be)
0xffffffff81390b8e is in pipe_write (fs/pipe.c:509).
warning: Source file is more recent than executable.
504                             kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
505                             do_wakeup = 0;
506                     }
507                     pipe->waiting_writers++;
508                     pipe_wait(pipe);
509                     pipe->waiting_writers--;
510             }
511     out:
512             __pipe_unlock(pipe);
513             if (do_wakeup) {

I rerun the test again (with a different address where it's stuck), there's
nothing better I can get from the debug info, it always points to pipe_wait,
disassembly points to:

   0xffffffff81390b71 <+417>:   jne    0xffffffff81390c23 <pipe_write+595>
   0xffffffff81390b77 <+423>:   test   %ecx,%ecx
   0xffffffff81390b79 <+425>:   jne    0xffffffff81390b95 <pipe_write+453>
   0xffffffff81390b7b <+427>:   addl   $0x1,0x110(%rbx)
   0xffffffff81390b82 <+434>:   mov    %rbx,%rdi
   0xffffffff81390b85 <+437>:   callq  0xffffffff813908c0 <pipe_wait>
   0xffffffff81390b8a <+442>:   subl   $0x1,0x110(%rbx)

(pipe_write+0x1ba == 0xffffffff81390b8a)